Rainbow Electronics MX7705 User Manual

General Description
The MX7705 low-power, 2-channel, serial-output ana­log-to-digital converter (ADC) includes a sigma-delta modulator with a digital filter to achieve 16-bit resolution with no missing codes. This ADC is pin compatible and software compatible with the AD7705. The MX7705 fea­tures an on-chip input buffer and programmable-gain amplifier (PGA). The device offers an SPI-/QSPI-/ MICROWIRE-compatible serial interface.
The MX7705 operates from a single 2.7V to 5.25V supply. The operating supply current is 320µA (typ) with a 3V supply. Power-down mode reduces the supply current to 2µA (typ).
Self-calibration and system calibration allow the MX7705 to correct for gain and offset errors. Excellent DC perfor­mance (±0.003% FSR INL) and low noise (650nV) make the MX7705 ideal for measuring low-frequency signals with a wide dynamic range. The device accepts fully dif­ferential bipolar/unipolar inputs. An internal input buffer allows for input signals with high source impedances. An on-chip digital filter, with a programmable cutoff and out­put data rate, processes the output of the sigma-delta modulator. The first notch frequency of the digital filter is chosen to provide 150dB rejection of common-mode 50Hz or 60Hz noise and 98dB rejection of normal-mode 50Hz or 60Hz noise. A PGA and digital filtering allow sig­nals to be directly acquired with little or no signal-condi­tioning requirements.
The MX7705 is available in 16-pin PDIP, SO, and TSSOP packages.
Applications
Industrial Instruments
Weigh Scales
Strain-Gauge Measurements
Loop-Powered Systems
Flow and Gas Meters
Medical Instrumentation
Pressure Transducers
Thermocouple Measurements
RTD Measurements
Features
Pin Compatible and Software Compatible with the
AD7705
16-Bit Sigma-Delta ADC
Two Fully Differential Input Channels
0.003% Integral Nonlinearity with No Missing Codes
Interface with Schmitt Triggers on Inputs
Internal Analog Input Buffers
PGA from 1 to 128
Single (2.7V to 3.6V) or (4.75V to 5.25V) Supply
Low Power
1mW (max), 3V Supply 2µA (typ) Power-Down Current
SPI-/QSPI-/MICROWIRE-Compatible 3-Wire Serial
Interface
16-Pin PDIP, SO, and TSSOP Packages
MX7705
16-Bit, Low-Power, 2-Channel,
Sigma-Delta ADC
________________________________________________________________ Maxim Integrated Products 1
Pin Configuration
Ordering Information
19-3051; Rev 0; 10/03
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
SPI/QSPI are trademarks of Motorola, Inc. MICROWIRE is a trademark of National Semiconductor Corp.
*Future product—contact factory for availability.
PART TEMP RANGE PIN-PACKAGE
MX7705EPE* -40°C to +85°C 16 PDIP
MX7705EWE* -40°C to +85°C 16 WIDE SO
MX7705EUE -40°C to +85°C 16 TSSOP
TOP VIEW
SCLK
CLKIN
CLKOUT
RESET
AIN2+
AIN1+
AIN1-
1
2
3
MX7705
4
CS
5
6
7
8
16
GND
15
V
DD
DIN
14
DOUT
13
DRDY
12
AIN2-
11
REF-
10
REF+
9
PDIP/SO/TSSOP
MX7705
16-Bit, Low-Power, 2-Channel, Sigma-Delta ADC
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VDD= 3V or 5V, GND = 0, V
REF+
= 1.225V for VDD= 3V and V
REF+
= 2.5V for VDD= 5V, V
REF-
= GND, external f
CLKIN
=
2.4576MHz, CLKDIV bit = 0, C
REF+
to GND = 0.1µF, C
REF
- to GND = 0.1µF, TA= T
MIN
to T
MAX
, unless otherwise noted.)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
VDDto GND..............................................................-0.3V to +6V
All Other Pins to GND.................................-0.3V to (V
DD
+ 0.3V)
Maximum Current Input into Any Pin ..................................50mA
Continuous Power Dissipation (T
A
= +70°C)
PDIP (derate 10.5mW/°C above +70°C)......................842mW
TSSOP (derate 9.4mW/°C above +70°C) ....................755mW
Wide SO (derate 9.5mW/°C above +70°C)..................762mW
Operating Temperature Range ..........................-40°C to +85°C
Storage Temperature Range .............................-60°C to +150°C
Junction Temperature......................................................+150°C
Lead Temperature (soldering, 10s) .................................+300°C
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
DC ACCURACY
Resolution (No Missing Codes) 16 Bits
Output Noise Tables 1, 3 µV
Integral Nonlinearity INL Gain = 1, unbuffered ±0.003 %FSR
Unipolar Offset Error After calibration (Note 1) µV
Unipolar Offset Drift (Note 2) 0.5 µV/°C
Bipolar Zero Error After calibration (Note 1) µV
Bipolar Zero Drift (Note 2)
Positive Full-Scale Error After calibration (Notes 1, 3) µV
Full-Scale Drift (Notes 2, 4) 0.5 µV/°C
Gain Error After calibration (Notes 1, 5) µV
Gain Drift (Notes 2, 6) 0.5
Bipolar Negative Full-Scale Error After calibration ±0.003 %FSR
Bipolar Negative Full-Scale Drift (Note 2)
ANALOG INPUTS (AIN1+, AIN1-, AIN2+, AIN2-)
AIN Differential Input Voltage Range (Note 7)
AIN Absolute Input Voltage Range (Note 8)
AIN DC Leakage Current Unselected input channel 1 nA
Gain = 1 to 4 0.5
Gain = 8 to 128 0.1
Gain = 1 to 4 1
Gain = 8 to 128 0.6
Unipolar input range 0
Bipolar input range
Unbuffered
Buffered
-V GAIN
GND -
30mV
GND +
50mV
/
REF
V
REF
GAIN
V
REF
GAIN
V
DD
30mV
V
DD
1.5V
/
/
+
-
µV/°C
ppm of FSR/°C
µV/°C
V
V
MX7705
16-Bit, Low-Power, 2-Channel,
Sigma-Delta ADC
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS (continued)
(VDD= 3V or 5V, GND = 0, V
REF+
= 1.225V for VDD= 3V and V
REF+
= 2.5V for VDD= 5V, V
REF-
= GND, external f
CLKIN
=
2.4576MHz, CLKDIV bit = 0, C
REF+
to GND = 0.1µF, C
REF
- to GND = 0.1µF, TA= T
MIN
to T
MAX
, unless otherwise noted.)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
AIN Input Capacitance
AIN Input Sampling Rate f
Input Common-Mode Rejection CMR
Normal-Mode 50Hz Rejection
Normal-Mode 60Hz Rejection
Common-Mode 50Hz Rejection
Common-Mode 60Hz Rejection
EXTERNAL REFERENCE (REF+, REF-)
REF Differential Input Voltage Range (Note 9)
RE F Ab sol ute Inp ut V ol tag e Rang e GND V
REF Input Capacitance Gain = 1 to 128 10 pF
REF Input Sampling Rate f
DIGITAL INPUTS (DIN, SCLK, CS, RESET)
Input High Voltage V
Input Low Voltage V
Input Hysteresis V
Input Current I
Input Capacitance 5pF
Gain = 1 34
Gain = 2 38
Gain = 4 45
Gain = 8 to 128 60
Gain = 1 to 128
s
VDD = 5V
V
DD
For filter notches of 25Hz, 50Hz, ±0.02 × f
For filter notches of 20Hz, 60Hz, ±0.02 × f
For filter notches of 25Hz, 50Hz, ±0.02 × f
For filter notches of 20Hz, 60Hz, ±0.02 × f
V
REF
HYST
IN
VDD = 4.75V to 5.25V 1.0 3.5
VDD = 2.7V to 3.6V 1.00 1.75
s
IH
VDD = 4.75V to 5.25V 0.8
IL
VDD = 2.7V to 3.6V 0.4 DIN, CS, RESET 250
SCLK 500
= 3V
NOTCH
NOTCH
NOTCH
NOTCH
Gain = 1 96
Gain = 2 105
Gain = 4 110
Gain = 8 to 128 130
Gain = 1 105
Gain = 2 110
Gain = 4 120
Gain = 8 to 128 130
f
CLKIN
64
98 dB
98 dB
150 dB
150 dB
f
CLKIN
64
2V
±1 µA
DD
pF
/
MHz
dB
V
V
/
MHz
V
mV
MX7705
16-Bit, Low-Power, 2-Channel, Sigma-Delta ADC
4 _______________________________________________________________________________________
ELECTRICAL CHARACTERISTICS (continued)
(VDD= 3V or 5V, GND = 0, V
REF+
= 1.225V for VDD= 3V and V
REF+
= 2.5V for VDD= 5V, V
REF-
= GND, external f
CLKIN
=
2.4576MHz, CLKDIV bit = 0, C
REF+
to GND = 0.1µF, C
REF
- to GND = 0.1µF, TA= T
MIN
to T
MAX
, unless otherwise noted.)
CLKIN INPUT
CLKIN Input High Voltage V
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
CLKIN Input Low Voltage V
CLKIN Input Current I
DIGITAL OUTPUTS (DOUT, DRDY, CLKOUT)
Output Voltage Low V
Output Voltage High V
Tri-State Leakage Current I
Tri-State Output Capacitance C
SYSTEM CALIBRATION
Full-Scale Calibration Range
CLKINH
CLKINL
CLKIN
OH
OUT
VDD = 4.75V to 5.25V 3.5
VDD = 2.7V to 3.6V 2.5
VDD = 4.75V to 5.25V 0.8
VDD = 2.7V to 3.6V 0.4
VDD = 5V
OL
VDD = 3V
VDD = 5V
VDD = 3V
DOUT only ±10 µA
L
DOUT only 9 pF
GAIN = selected PGA gain (1 to 128) (Note 10)
DOUT and DRDY, I
= 800µA
SINK
CLKOUT, I
= 10µA
SINK
DOUT and DRDY, I
= 100µA
SINK
CLKOUT, I
= 10µA
SINK
DOUT and DRDY, I
CLKOUT, I
DOUT and DRDY, I
CLKOUT, I
SOURCE
SOURCE
SOURCE
SOURCE
= 200µA
= 10µA
= 100µA
= 10µA
4.0
4.0
V
DD
0.6V
V
DD
0.6V
-1.05 × V
REF
GAIN
-
-
/
±10 µA
0.4
0.4
0.4
0.4
1.05 × V
/
REF
GAIN
V
V
V
V
V
-1.05 × V
REF
GAIN
0.8 ×
V
REF
GAIN
/
/
Offset Calibration Range
Input Span
GAIN = selected PGA gain (1 to 128) (Note 10)
GAIN = selected PGA gain (1 to 128) (Notes 10, 11)
1.05 × V
REF
GAIN
2.1 ×
V
REF
GAIN
/
V
/
V
MX7705
16-Bit, Low-Power, 2-Channel,
Sigma-Delta ADC
_______________________________________________________________________________________ 5
ELECTRICAL CHARACTERISTICS (continued)
(VDD= 3V or 5V, GND = 0, V
REF+
= 1.225V for VDD= 3V and V
REF+
= 2.5V for VDD= 5V, V
REF-
= GND, external f
CLKIN
=
2.4576MHz, CLKDIV bit = 0, C
REF+
to GND = 0.1µF, C
REF
- to GND = 0.1µF, TA= T
MIN
to T
MAX
, unless otherwise noted.)
POWER REQUIREMENTS
Power-Supply Voltage V
Power-Supply Current (Note 12) I
Power-Supply Rejection Ratio PSRR
EXTERNAL CLOCK TIMING SPECIFICATIONS
CLKIN Frequency f
Duty Cycle 40 60 %
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
DD
Unbuffered f
CLKIN
gain =1 to 128
Buffered, f
CLKIN
gain =1 to 128
Unbuffered, f
CLKIN
DD
Buffered, f
CLKIN
Power-down mode (Note 13)
VDD = 4.75V to 5.25V (Note 14)
V
DD
CLKIN
(Note 15) 400 2500 kHz
=1MHz,
=1MHz,
= 2.4576MHz
= 2.4576MHz
= 2.7V to 3.6V (Note 14)
VDD = 5V 0.45
V
DD
VDD = 5V 0.7
V
DD
VDD = 5V, gain = 1 to 4
VDD = 5V, gain = 8 to 128
VDD = 3V, gain = 1 to 4
V
DD
gain = 8 to 128
VDD = 5V, gain = 1 to 4
VDD = 5V, gain = 8 to 128
VDD = 3V, gain = 1 to 4
V
DD
gain = 8 to 128
VDD = 5V 16
V
DD
= 3V 0.32
= 3V 0.6
= 3V,
= 3V,
= 3V 8
2.70 5.25 V
0.6
0.85
0.4
0.6
0.9
1.3
0.7
1.1
mA
µA
dB
MX7705
16-Bit, Low-Power, 2-Channel, Sigma-Delta ADC
6 _______________________________________________________________________________________
TIMING CHARACTERISTICS
(VDD= 3V or 5V, GND = 0, V
REF+
= 1.225V for VDD= 3V and V
REF+
= 2.5V for VDD= 5V, V
REF-
= GND, external f
CLKIN
=
2.4576MHz, CLKDIV bit = 0, C
REF+
to GND = 0.1µF, C
REF
- to GND = 0.1µF, TA= T
MIN
to T
MAX
, unless otherwise noted.) (Note 16)
(Figures 8, 9)
DRDY High Time
Reset Pulse-Width Low 100 ns
DRDY Fall to CS Fall Setup Time t CS Fall to SCLK Rise Setup Time t
SCLK Fall to DOUT Valid Delay t
SCLK Pulse-Width High t
SCLK Pulse-Width Low t CS Rise to SCLK Rise Hold Time t
Bus Relinquish Time After SCLK Rising Edge
SCLK Fall to DRDY Rise Delay t
DIN to SCLK Setup Time t
DIN to SCLK Hold Time t
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
1
2
VDD = 4.75V to 5.25V 0 80
3
VDD = 2.7V to 3.6V 0 100
4
5
6
t
10
VDD = 4.75V to 5.25V 60
7
VDD = 2.7V to 3.6V 100
8
9
500 /
f
CLKIN
120 ns
100 ns
100 ns
30 ns
20 ns
0ns
0ns
100 ns
s
ns
ns
MX7705
16-Bit, Low-Power, 2-Channel,
Sigma-Delta ADC
_______________________________________________________________________________________ 7
Note 1: These errors are in the order of the conversion noise shown in Tables 1 and 3. This applies after calibration at the given
temperature.
Note 2: Recalibration at any temperature removes these drift errors. Note 3: Positive full-scale error includes zero-scale errors (unipolar offset error or bipolar zero error) and applies to both unipolar
and bipolar input ranges.
Note 4: Full-scale drift includes zero-scale drift (unipolar offset drift or bipolar zero drift) and applies to both unipolar and bipolar
input ranges.
Note 5: Gain error does not include zero-scale errors. It is calculated as (full-scale error - unipolar offset error) for unipolar ranges,
and (full-scale error - bipolar zero error) for bipolar ranges.
Note 6: Gain-error drift does not include unipolar offset drift or bipolar zero drift. Effectively, it is the drift of the part if only zero-
scale calibrations are performed.
Note 7: The analog input voltage range on AIN+ is given with respect to the voltage on AIN- on the MX7705. Note 8: This common-mode voltage range is allowed, provided that the input voltage on analog inputs does not go more positive
than (V
DD
+ 30mV) or more negative than (GND - 30mV). Parts are functional with voltages down to (GND - 200mV), but
with increased leakage at high temperature.
Note 9: The REF differential voltage, V
REF
, is the voltage on REF+ referenced to REF- (V
REF
= V
REF+
- V
REF-)
.
Note 10: Guaranteed by design. Note 11: These calibration and span limits apply, provided that the absolute voltage on the analog inputs does not exceed (V
DD
+ 30mV) or go more negative than (GND - 30mV). The offset calibration limit applies to both the unipolar zero point and the bipolar zero point.
Note 12: When using a crystal or ceramic resonator across the CLKIN and CLKOUT as the clock source for the device, the supply
current and power dissipation varies depending on the crystal or resonator type. Supply current is measured with the digi­tal inputs connected to 0 or V
DD
, CLKIN connected to an external clock source, and CLKDIS = 1.
Note 13: If the external master clock continues to run in power-down mode, the power-down current typically increases to 67µA at
3V. When using a crystal or ceramic resonator across the CLKIN and CLKOUT as the clock source for the device, the clock generator continues to run in power-down mode and the power dissipation depends on the crystal or resonator type (see the Power-Down Modes section).
Note 14: Measured at DC and applied in the selected passband. PSRR at 50Hz exceeds 120dB with filter notches of 25Hz or 50Hz.
PSRR at 60Hz exceeds 120dB with filter notches of 20Hz or 60Hz. PSRR depends on both gain and V
DD
.
Note 15: Provide f
CLKIN
whenever the MX7705 is not in power-down mode. If no clock is present, the device can draw higher than
specified current and can possibly become uncalibrated.
Note 16: All input signals are specified with t
r
= tf= 5ns (10% to 90% of VDD) and timed from a voltage level of 1.6V.
TIMING CHARACTERISTICS (continued)
(VDD= 3V or 5V, GND = 0, V
REF+
= 1.225V for VDD= 3V and V
REF+
= 2.5V for VDD= 5V, V
REF-
= GND, external f
CLKIN
=
2.4576MHz, CLKDIV bit = 0, C
REF+
to GND = 0.1µF, C
REF
- to GND = 0.1µF, TA= T
MIN
to T
MAX
, unless otherwise noted.) (Note 16)
(Figures 8, 9)
GAIN
19086
27878
48485
8 to 128 91 93
PSRR (dB) (V
= 5V)
DD
PSRR (dB) (VDD = 3V)
MX7705
16-Bit, Low-Power, 2-Channel, Sigma-Delta ADC
8 _______________________________________________________________________________________
Table 1. Output RMS Noise vs. Gain and Output Data Rate (VDD= 5V)
FILTER FIRST
NOTCH AND
OUTPUT DATA RATE
BUFFERED (f
UNBUFFERED (f
BUFFERED (f
UNBUFFERED (f
CLKIN
20Hz 5.24Hz 4.44 2.28 1.29 0.79 0.70 0.70 0.64 0.63
25Hz 6.55Hz 5.11 2.79 1.55 0.92 0.81 0.80 0.73 0.74
100Hz 26.2Hz 102.35 49.59 23.04 11.78 6.32 3.63 2.25 2.24
200Hz 52.4Hz 586.93 272.83 224.79 70.78 33.94 17.57 9.14 9.22
20Hz 5.24Hz 4.32 2.50 1.35 0.81 0.73 0.70 0.64 0.64
25Hz 6.55Hz 5.16 2.85 1.63 0.96 0.83 0.81 0.74 0.74
100Hz 26.2Hz 105.78 49.86 24.67 12.16 6.42 3.80 2.22 2.23
200Hz 52.4Hz 526.60 260.51
CLKIN
50Hz 13.1Hz 3.53 1.86 1.09 0.73 0.72 0.71 0.67 0.66
60Hz 15.72Hz 4.41 2.23 1.29 0.83 0.79 0.77 0.72 0.73
250Hz 65.5Hz 99.66 46.85 16.98 12.48 6.38 3.78 2.32 2.35
500Hz 131Hz 608.86 288.39 110.80 67.51 36.75 17.98 9.43 9.40
50Hz 13.1Hz 3.65 1.94 1.17 0.79 0.70 0.69 0.66 0.65
60Hz 15.72Hz 4.56 2.41 1.32 0.87 0.80 0.79 0.71 0.74
250Hz 65.5Hz 101.56 49.64 25.39 12.92 6.65 3.69 2.36 2.36
500Hz 131Hz 556.06 278.91 142.88 74.78 35.41 18.99 9.80 9.44
CLKIN
CLKIN
-3dB FREQUENCY
1 2 4 8 16 32 64 128
= 1MHz)
= 1MHz)
= 2.4576MHz)
= 2.4576MHz)
TYPICAL OUTPUT RMS NOISE (µV)
GAIN
132.16
67.25 34.09 18.20 8.95 9.08
MX7705
16-Bit, Low-Power, 2-Channel,
Sigma-Delta ADC
_______________________________________________________________________________________ 9
Table 2. Peak-to-Peak Resolution vs. Gain and Output Data Rate (VDD= 5V)
FILTER FIRST
NOTCH AND
OUTPUT DATA RATE
BUFFERED (f
UNBUFFERED (f
BUFFERED (f
UNBUFFERED (f
CLKIN
20Hz 5.24Hz 16 16 16 16 16 15 14 13
25Hz 6.55Hz 16 16 16 16 16 15 14 13
100Hz 26.2Hz 12 12 12 12 12 12 12 11
200Hz 52.4Hz 10 10 10 10 10 10 10 9
CLKIN
20Hz 5.24Hz 16 16 16 16 16 15 14 13
25Hz 6.55Hz 16 16 16 16 16 15 14 13
100Hz 26.2Hz 12 12 12 12 12 12 12 11
200Hz 52.4Hz 10 10 10 10 10 10 10 9
CLKIN
50Hz 13.1Hz 16 16 16 16 16 15 14 13
60Hz 15.72Hz 16 16 16 16 16 15 14 13
250Hz 65.5Hz 12 12 13 12 12 12 12 11
500Hz 131Hz 10 10 11 10 10 10 10 9
CLKIN
50Hz 13.1Hz 16 16 16 16 16 15 14 13
60Hz 15.72Hz 16 16 16 16 16 15 14 13
250Hz 65.5Hz 12 12 12 12 12 12 12 11
500Hz 131Hz 10 10 10 10 10 10 10 9
-3dB FREQUENCY
1 2 4 8 16 32 64 128
= 1MHz)
= 1MHz)
= 2.4576MHz)
= 2.4576MHz)
TYPICAL PEAK-TO-PEAK RESOLUTION (BITS)
GAIN
MX7705
16-Bit, Low-Power, 2-Channel, Sigma-Delta ADC
10 ______________________________________________________________________________________
Table 3. Output RMS Noise vs. Gain and Output Data Rate (VDD= 3V)
FILTER FIRST
NOTCH AND
OUTPUT DATA RATE
BUFFERED (f
UNBUFFERED (f
BUFFERED (f
UNBUFFERED (f
CLKIN
20Hz 5.24Hz 3.52 1.84 2.19 0.73 0.66 0.62 0.62 0.62
25Hz 6.55Hz 4.24 2.23 1.19 0.84 0.74 0.69 0.69 0.69
100Hz 26.2Hz 50.36 25.12 12.06 6.04 3.38 2.23 1.70 1.69
200Hz 52.4Hz 268.02 175.98 65.77 34.89 16.73 8.76 4.70 4.70
20Hz 5.24Hz 3.58 1.92 1.13 0.72 0.66 0.64 0.61 0.62
25Hz 6.55Hz 4.16 2.27 1.27 0.83 0.74 0.70 0.69 0.67
100Hz 26.2Hz 50.48 23.89 12.10 5.90 3.26 0.70 1.66 1.63
200Hz 52.4Hz 256.43 135.78 65.62 33.18 16.65 8.47 4.66 4.68
CLKIN
50Hz 13.1Hz 2.84 1.68 2.00 0.71 0.67 0.65 0.63 0.61
60Hz 15.72Hz 3.27 1.84 1.12 0.78 0.75 0.70 0.68 0.67
250Hz 65.5Hz 47.90 24.43 12.56 6.48 3.45 2.32 1.64 1.66
500Hz 131Hz 281.03 104.19 69.58 34.59 17.44 9.20 5.16 4.92
50Hz 13.1Hz 3.04 1.74 1.03 0.72 0.64 0.64 0.62 0.63
60Hz 15.72Hz 3.35 1.80 1.13 0.81 0.73 0.69 0.67 0.68
250Hz 65.5Hz 49.63 23.82 13.03 6.23 3.42 2.22 1.68 1.65
500Hz 131Hz 279.13 134.82 69.47 35.42 17.47 9.55 4.90 5.18
CLKIN
CLKIN
-3dB FREQUENCY
1248163264128
= 1MHz)
= 1MHz)
= 2.4576MHz)
= 2.4576MHz)
TYPICAL OUTPUT RMS NOISE (µV)
GAIN
MX7705
16-Bit, Low-Power, 2-Channel,
Sigma-Delta ADC
______________________________________________________________________________________ 11
Table 4. Peak-to-Peak Resolution vs. Gain and Output Data Rate (VDD= 3V)
Typical Operating Characteristics
(VDD= 3V or 5V, V
REF+
= 1.225V for VDD= 3V, V
REF+
= 2.5V for VDD= 5V, V
REF
- = GND, TA= +25°C, unless otherwise noted.)
FILTER FIRST
NOTCH AND
OUTPUT DATA RATE
BUFFERED (f
CLKIN
-3dB FREQUENCY
1 2 4 8 16 32 64 128
= 1MHz)
TYPICAL PEAK-TO-PEAK RESOLUTION (BITS)
GAIN
20Hz 5.24Hz 16 16 16 16 15 14 13 12
25Hz 6.55Hz 16 16 16 16 15 14 13 12
100Hz 26.2Hz 12 12 12 12 12 12 12 11
200Hz 52.4Hz 10 10 10 10 10 10 10 9
UNBUFFERED (f
CLKIN
= 1MHz)
20Hz 5.24Hz 16 16 16 16 15 14 13 12
25Hz 6.55Hz 16 16 16 16 15 14 13 12
100Hz 26.2Hz 12 12 12 12 12 14 12 11
200Hz 52.4Hz 10 10 10 10 10 10 10 9
BUFFERED (f
= 2.4576MHz)
CLKIN
50Hz 13.1Hz 16 16 16 16 15 14 13 12
60Hz 15.72Hz 16 16 16 16 15 14 13 12
250Hz 65.5Hz 12 12 12 12 12 12 11 11
500Hz 131Hz 10 11 10 10 10 10 10 9
UNBUFFERED (f
= 2.4576MHz)
CLKIN
50Hz 13.1Hz 16 16 16 16 15 14 13 12
60Hz 15.72Hz 16 16 16 16 15 14 13 12
250Hz 65.5Hz 12 12 12 12 12 12 11 11
500Hz 131Hz 10 10 10 10 10 10 10 9
32776
32774
32772
32770
32768
32766
CODE READ
32764
32762
32760
32758
32756
TYPICAL OUTPUT NOISE
VDD = 5V,
= 2.5V
V
REF
0 2000
READING NUMBER
GAIN = 128 ODR = 60Hz RMS NOISE = 1.3µV
16001200800400
MX7705 toc01
HISTOGRAM OF TYPICAL OUTPUT NOISE
400
VDD = 5V, V GAIN = 128 ODR = 60Hz
300
200
OCCURENCE
100
0
32760
32761
REF
32762
= 2.5V
32763
32764
32765
CODE
RMS NOISE = 1.3µV
32768
32767
32766
32769
32770
32771
32772
MX7705 toc02
32773
OFFSET ERROR vs. SUPPLY VOLTAGE (3V)
0.0015
0.0010
0.0005
-0.0005 OFFSET ERROR (%FSR)
-0.0010
-0.0015
VDD = 3V
0
2.70 3.60 SUPPLY VOLTAGE (V)
MX7705 toc03
3.453.303.153.002.85
MX7705
16-Bit, Low-Power, 2-Channel, Sigma-Delta ADC
12 ______________________________________________________________________________________
Typical Operating Characteristics (continued)
(VDD= 3V or 5V, V
REF+
= 1.225V for VDD= 3V, V
REF+
= 2.5V for VDD= 5V, V
REF
- = GND, TA= +25°C, unless otherwise noted.)
OFFSET ERROR vs. SUPPLY VOLTAGE (5V)
0.003 VDD = 5V
0.002
0.001
MX7705 toc04
OFFSET ERROR vs. TEMPERATURE
0.003
0.002
0.001
VDD = 5V
GAIN ERROR vs. SUPPLY VOLTAGE (3V)
0.0015 VDD = 3V
MX7705 toc05
0.0010
0.0005
MX7705 toc06
0
-0.001
OFFSET ERROR (%FSR)
-0.002
-0.003
4.75 5.25 SUPPLY VOLTAGE (V)
GAIN ERROR vs. SUPPLY VOLTAGE (5V)
0.003 VDD = 5V
0.002
0.001
0
-0.001
GAIN ERROR (%FSR)
-0.002
-0.003
4.75 5.25
5.155.054.954.85
SUPPLY VOLTAGE (V)
0
-0.001
OFFSET ERROR (%FSR)
-0.002
-0.003
VDD = 3V
-40 85 TEMPERATURE (°C)
MX7705 toc07
-0.001
GAIN ERROR (%FSR)
-0.002
-0.003
-0.004
5.155.054.954.85
-0.005
0
-0.0005
GAIN ERROR (%FSR)
-0.0010
603510-15
-0.0015
2.70 3.60
GAIN ERROR vs. TEMPERATURE
0.005
0.004
0.003
0.002
0.001
VDD = 3V
0
VDD = 5V
-40 85 TEMPERATURE (°C)
3.453.303.153.002.85
SUPPLY VOLTAGE (V)
MX7705 toc08
603510-15
MX7705
16-Bit, Low-Power, 2-Channel,
Sigma-Delta ADC
______________________________________________________________________________________ 13
Typical Operating Characteristics (continued)
(VDD= 3V or 5V, V
REF+
= 1.225V for VDD= 3V, V
REF+
= 2.5V for VDD= 5V, V
REF
- = GND, TA= +25°C, unless otherwise noted.)
A: BUFFERED MODE f
CLKIN
GAIN = 8 TO 128
D: UNBUFFERED MODE f
CLKIN
GAIN = 1 TO 128
SUPPLY CURRENT vs. SUPPLY VOLTAGE (3V)
0.6
A
0.5
B
0.4 C
SUPPLY CURRENT (mA)
D
0.3 E
0.2
2.70 3.60 SUPPLY VOLTAGE (V)
= 2.4576MHz,
= 2.4576MHz,
B: BUFFERED MODE
= 2.4576MHz,
f
CLKIN
GAIN = 1 TO 4
E: UNBUFFERED MODE
= 1MHz,
f
CLKIN
GAIN = 1 TO 128
VDD = 3V
3.453.303.153.002.85
C: BUFFERED MODE f
CLKIN
GAIN = 1 TO 128
MX7705 toc09
= 1MHz,
SUPPLY CURRENT vs. SUPPLY VOLTAGE (5V)
0.65 A
0.55
0.45 C
SUPPLY CURRENT (mA)
0.35 E
0.25
4.75 5.25
A: BUFFERED MODE f
= 2.4576MHz,
CLKIN
GAIN = 8 TO 128
D: UNBUFFERED MODE
= 2.4576MHz,
f
CLKIN
GAIN = 1 TO 128
VDD = 5V
D
SUPPLY VOLTAGE (V)
B: BUFFERED MODE
= 2.4576MHz,
f
CLKIN
GAIN = 1 TO 4
E: UNBUFFERED MODE
= 1MHz,
f
CLKIN
GAIN = 1 TO 128
B
5.155.054.954.85
C: BUFFERED MODE
= 1MHz,
f
CLKIN
GAIN = 1 TO 128
MX7705 toc10
SUPPLY CURRENT vs. TEMPERATURE (3V)
0.6
A
0.5
C
0.4
SUPPLY CURRENT (mA)
0.3
0.2
-40 85
A: BUFFERED MODE
= 2.4576MHz,
f
CLKIN
GAIN = 8 TO 128
D: UNBUFFERED MODE
= 2.4576MHz,
f
CLKIN
GAIN = 1 TO 128
D
E
TEMPERATURE (°C)
B: BUFFERED MODE f
CLKIN
GAIN = 1 TO 4
E: UNBUFFERED MODE f
CLKIN
GAIN = 1 TO 128
= 2.4576MHz,
= 1MHz,
VDD = 3V
B
603510-15
C: BUFFERED MODE
= 1MHz,
f
CLKIN
GAIN = 1 TO 128
MX7705 toc11
SUPPLY CURRENT vs. TEMPERATURE (5V)
0.65 A
0.55
C
0.45
SUPPLY CURRENT (mA)
0.35
E
0.25
-40 85
A: BUFFERED MODE
= 2.4576MHz,
f
CLKIN
GAIN = 8 TO 128
D: UNBUFFERED MODE
= 2.4576MHz,
f
CLKIN
GAIN = 1 TO 128
VDD = 5V
D
TEMPERATURE (°C)
B: BUFFERED MODE
= 2.4576MHz,
f
CLKIN
GAIN = 1 TO 4
E: UNBUFFERED MODE
= 1MHz,
f
CLKIN
GAIN = 1 TO 128
B
603510-15
C: BUFFERED MODE
= 1MHz,
f
CLKIN
GAIN = 1 TO 128
MX7705 toc12
MX7705
16-Bit, Low-Power, 2-Channel, Sigma-Delta ADC
14 ______________________________________________________________________________________
Typical Operating Characteristics (continued)
(VDD= 3V or 5V, V
REF+
= 1.225V for VDD= 3V, V
REF+
= 2.5V for VDD= 5V, V
REF
- = GND, TA= +25°C, unless otherwise noted.)
SUPPLY CURRENT vs. f
0.6 VDD = 3V
A
0.5
0.4
C
SUPPLY CURRENT (mA)
0.3
E
0.2
A: BUFFERED MODE CLK = 1, GAIN = 128
D: UNBUFFERED MODE CLK = 1, GAIN = 1, 128
SUPPLY CURRENT vs. GAIN (3V)
0.6 VDD = 3V
0.5
D
f
(MHz)
CLKIN
B: BUFFERED MODE CLK = 1, GAIN = 1
E: UNBUFFERED MODE CLK = 0, GAIN = 1, 128
A
(3V)
CLKIN
B
2.22.01.81.61.41.21.00.80.60.4 2.6
2.4
C: BUFFERED MODE CLK = 0, GAIN = 1, 128
B
MX7705 toc13
MX7705 toc15
SUPPLY CURRENT vs. f
0.65 VDD = 5V
0.55
0.45
SUPPLY CURRENT (mA)
0.35
0.25
A: BUFFERED MODE CLK = 1, GAIN = 128
D: UNBUFFERED MODE CLK = 1, GAIN = 1, 128
A
C
SUPPLY CURRENT vs. GAIN (5V)
0.65 VDD = 5V
0.55
D
E
f
(MHz)
CLKIN
B: BUFFERED MODE CLK = 1, GAIN = 1
E: UNBUFFERED MODE CLK = 0, GAIN = 1, 128
A
B
(5V)
CLKIN
B
2.22.01.81.61.41.21.00.80.60.4 2.6
2.4
C: BUFFERED MODE CLK = 0, GAIN = 1, 128
MX7705 toc14
MX7705 toc16
0.4
SUPPLY CURRENT (mA)
0.3
0.2
A: BUFFERED MODE CLK = 1, CLKDIV = 1,
= 2.4576MHz
f
CLKIN
D: UNBUFFERED MODE CLK = 1, CLKDIV = 1,
= 2.4576MHz
f
CLKIN
C
D
F
GAIN
B: BUFFERED MODE CLK = 1, CLKDIV = 0,
= 2.4576MHz
f
CLKIN
E: UNBUFFERED MODE CLK = 1, CLKDIV = 0,
= 2.4576MHz
f
CLKIN
E
32168421 128
C: BUFFERED MODE CLK = 0, CLKDIV = 0, f
F: UNBUFFERED MODE CLK = 0, CLKDIV = 0, f
64
CLKIN
CLKIN
= 1MHz
= 1MHz
0.45
SUPPLY CURRENT (mA)
0.35
0.25
A: BUFFERED MODE CLK = 1, CLKDIV = 0,
= 2.4576MHz
f
CLKIN
D: UNBUFFERED MODE CLK = 1, CLKDIV = 1,
= 2.4576MHz
f
CLKIN
C
D
E
F
GAIN
B: BUFFERED MODE CLK = 1, CLKDIV = 1,
= 2.4576MHz
f
CLKIN
E: UNBUFFERED MODE CLK = 1, CLKDIV = 0,
= 2.4576MHz
f
CLKIN
32168421 128
C: BUFFERED MODE CLK = 0, CLKDIV = 0, f
F: UNBUFFERED MODE CLK = 0, CLKDIV = 0, f
64
CLKIN
CLKIN
= 1MHz
= 1MHz
MX7705
16-Bit, Low-Power, 2-Channel,
Sigma-Delta ADC
______________________________________________________________________________________ 15
Typical Operating Characteristics (continued)
(VDD= 3V or 5V, V
REF+
= 1.225V for VDD= 3V, V
REF+
= 2.5V for VDD= 5V, V
REF
- = GND, TA= +25°C, unless otherwise noted.)
POWER-DOWN SUPPLY CURRENT
vs. SUPPLY VOLTAGE (3V)
100
VDD = 3V
80
60
40
20
POWER-DOWN SUPPLY CURRENT (nA)
0
2.70 3.60 SUPPLY VOLTAGE (V)
POWER-DOWN SUPPLY CURRENT
vs. SUPPLY VOLTAGE (5V)
200
VDD = 5V
MX7705 toc17
180
160
140
120
POWER-DOWN SUPPLY CURRENT (nA)
3.453.303.153.002.85
100
4.75 5.25 SUPPLY VOLTAGE (V)
EXTERNAL OSCILLATOR STARTUP TIME
POWER-DOWN SUPPLY CURRENT
vs. TEMPERATURE
300
MX7705 toc18
5.155.054.954.85
MX7705 toc20
250
200
150
100
50
POWER-DOWN SUPPLY CURRENT (µA)
0
-40 85
VDD = 5V
VDD = 3V
TEMPERATURE (°C)
MX7705 toc19
603510-15
V
DD
5V/div
4.9152MHz CRYSTAL
2.4576MHz CRYSTAL
2ms/div
CLKOUT 5V/div
CLKOUT 5V/div
MX7705
16-Bit, Low-Power, 2-Channel, Sigma-Delta ADC
16 ______________________________________________________________________________________
Pin Description
PIN NAME FUNCTION
1 SCLK
2 CLKIN
3 CLKOUT
4 CS
5 RESET Active-Low Reset Input. Drive RESET low to reset the MX7705 to power-on reset status.
6 AIN2+ Channel 2 Positive Differential Analog Input
7 AIN1+ Channel 1 Positive Differential Analog Input
8 AIN1- Channel 1 Negative Differential Analog Input
9 REF+ Positive Differential Reference Input
10 REF- Negative Differential Reference Input
11 AIN2- Channel 2 Negative Differential Analog Input
12 DRDY
13 DOUT
14 DIN Serial Data Input. Data on DIN is clocked in on the rising edge of SCLK when CS is low.
15 V
16 GND Ground
DD
Serial Clock Input. Apply an external serial clock to transfer data to and from the device at data rates of up to 5MHz.
Clock Input. Connect a crystal/resonator between CLKIN and CLKOUT, or drive CLKIN externally with a CMOS-compatible clock source with CLKOUT left unconnected.
Clock Output. Connect a crystal/resonator between CLKIN and CLKOUT. When enabled, CLKOUT provides a CMOS-compatible, inverted clock output. Set CLKDIS = 0 in the clock register to enable CLKOUT. Set CLKDIS = 1 in the clock register to disable CLKOUT to conserve power.
Active-Low Chip-Select Input. CS selects the active device in systems with more than one device on the serial bus. Drive CS low to clock data in on DIN and to clock data out on DOUT. When CS is high, DOUT is high impedance. Connect CS to GND for 3-wire operation.
Active-Low Data-Ready Output. DRDY goes low when a new conversion result is available in the data register. When a read-operation of a full output word completes, DRDY returns high.
Serial Data Output. DOUT outputs serial data from the data register. DOUT changes on the falling edge of SCLK and is valid on the rising edge of SCLK. When CS is high, DOUT is high impedance.
Power Input
Detailed Description
The MX7705 low-power, 2-channel, serial-output ADC uses a sigma-delta modulator with a digital filter to achieve 16-bit resolution with no missing codes. The device includes a PGA, an on-chip input buffer, and a bidirectional communications port. The MX7705 oper­ates with a single 2.7V to 5.25V supply.
Fully differential inputs, an internal input buffer, and an on-chip PGA (gain = 1 to 128) allow low-level signals to be directly measured, minimizing the requirements for external signal conditioning. Self-calibration corrects for gain and offset errors. A programmable digital filter allows for the selection of the output data rate and first­notch frequency from 20Hz to 500Hz.
The bidirectional serial SPI-/QSPI-/MICROWIRE-compati­ble interface consists of four digital control lines (SCLK, CS, DOUT, and DIN) and provides an easy interface to microcontrollers (µCs). Connect CS to GND to configure the MX7705 for 3-wire operation.
Analog Inputs
The MX7705 accepts four analog inputs (AIN1+, AIN1-, AIN2+, and AIN2-) in buffered or unbuffered mode. Use Table 8 to select the positive and negative input pair for a fully differential channel. The input buffer iso­lates the inputs from the capacitive load presented by the PGA/modulator, allowing for high source-imped­ance analog transducers. The value of the BUF bit in the setup register (see the Setup Register section) deter­mines whether the input buffer is enabled or disabled.
Internal protection diodes, which clamp the analog input to VDDand/or GND, allow the input to swing from (GND - 0.3V) to (VDD+ 0.3V), without damaging the device. If the analog input exceeds 300mV beyond the supplies, limit the input current to 10mA.
Input Buffers
When the analog input buffer is disabled, the analog input drives a typical 7pF (gain = 1) capacitor, C
TOTAL
, in series with the 7ktypical on-resistance of the track and hold (T/H) switch (Figure 1). C
TOTAL
is comprised
of the sampling capacitor, C
SAMP
, and the stray capac-
itance, C
STRAY
. During the conversion, C
SAMP
charges to (AIN+ - AIN-). The gain determines the value of C
SAMP
(Table 5).
MX7705
16-Bit, Low-Power, 2-Channel,
Sigma-Delta ADC
______________________________________________________________________________________ 17
Functional Diagram
MX7705
BUFFER
AIN1+
AIN1-
AIN2+
AIN2-
REF+
REF-
SWITCHING
NETWORK
S1
S2
BUFFER
S1 AND S2 ARE OPEN IN
BUFFERED MODE AND CLOSED
IN UNBUFFERED MODE
PGA
DIVIDER
2ND-ORDER
SIGMA-DELTA
MODULATOR
SERIAL INTERFACE,
REGISTERS,
CLOCK
GENERATOR
DIGITAL
FILTER
AND
CONTROL
CLKIN
CLKOUT
V
DD
GND
CS SCLK DIN DOUT DRDY RESET
MX7705
To minimize gain errors in unbuffered mode, select a source impedance less than the maximum values shown in Figures 2 and 3. These are the maximum external resistance/capacitance combinations allowed before gain errors greater than 1 LSB are introduced in unbuffered mode.
Enable the internal input buffer for a high source imped­ance. This isolates the inputs from the sampling capaci­tor and reduces the sampling-related gain error. When using the internal buffer, limit the absolute input voltage range to (V
GND
+ 50mV) to (VDD- 1.5V). Set gain and common-mode voltage range properly to minimize lin­earity errors.
Input Voltage Range
In unbuffered mode, the absolute analog input voltage range is from (GND - 30mV) to (VDD+ 30mV) (see the Electrical Characteristics). In buffered mode, the ana­log input voltage range is reduced to (GND + 50mV) to (VDD- 1.5V). In both buffered and unbuffered modes, the differential analog input range (V
AIN+
- V
AIN-
) decreases at higher gains (see the Programmable-Gain Amplifier and the Unipolar and Bipolar Modes sections).
Reference
The MX7705 provides differential inputs, REF+ and REF-, for an external reference voltage. Connect the external reference directly across REF+ and REF- to obtain the differential reference voltage, V
REF
. The common-mode
voltage range for V
REF+
and V
REF-
is between GND
and V
DD
. For specified operation, the nominal voltage,
V
REF(VREF+
- V
REF-
), is 1.225V for VDD= 4.75V to
5.25V and 2.5V for VDD= 2.7V to 3.6V.
The MX7705 samples REF+ and REF- at f
CLKIN
/ 64
(CLKDIV = 0) or f
CLKIN
/ 128 (CLKDIV = 1) with an internal 10pF (typ for gain = 1) sampling capacitor in series with a 7k(typ) switch on-resistance.
Programmable-Gain Amplifier
A PGA provides selectable levels of gain: 1, 2, 4, 8, 16, 32, 64, and 128. Bits G0, G1, and G2 in the setup reg­ister control the gain (Table 9). As the gain increases, the value of the input sampling capacitor, C
SAMP
, also increases (Table 5). The dynamic load presented to the analog inputs increases with clock frequency and gain in unbuffered mode (see the Input Buffers section and Figure 1).
16-Bit, Low-Power, 2-Channel, Sigma-Delta ADC
18 ______________________________________________________________________________________
Figure 1. Unbuffered Analog Input Structure
Figure 2. Maximum External Resistance vs. Maximum External Capacitance for Unbuffered Mode (1MHz)
Figure 3. Maximum External Resistance vs. Maximum External Capacitance for Unbuffered Mode (2.4576MHz)
AIN(+)
AIN(-)
(7k TYP)
R
SW
IMPEDANCE
C
(7pF TYP FOR GAIN = 1)
TOTAL
C
TOTAL
V
BIAS
100
10
1
EXTERNAL RESISTANCE (k)
0.1 1 10 100 1000 10,000
100
10
1
EXTERNAL RESISTANCE (k)
0.1 1 10 100 1000 10,000
GAIN = 1
GAIN = 2
GAIN = 4
GAIN = 8 TO 128
EXTERNAL CAPACITANCE (pF)
GAIN = 1
GAIN = 2
GAIN = 4
GAIN = 8 TO 128
EXTERNAL CAPACITANCE (pF)
HIGH
= C
SAMP
+ C
STRAY
Increasing the gain increases the resolution of the ADC (LSB size decreases), but reduces the differential input voltage range. Calculate 1 LSB in unipolar mode using the following equation:
where V
REF
= V
REF+
- V
REF-.
For a gain of one and V
REF
= 2.5V, the full-scale volt­age in unipolar mode is 2.5V and 1 LSB 38.1µV. For a gain of four, the full-scale voltage in unipolar mode is
0.625V (V
REF
/ GAIN) and 1 LSB 9.5µV. The differen-
tial input voltage range in this example reduces from
2.5V to 0.625V, and the resolution increases, since the LSB size decreased from 38.1µV to 9.5µV.
Calculate 1 LSB in bipolar mode using the following equation:
where V
REF
= V
REF+
- V
REF-.
Unipolar and Bipolar Modes
The B/U bit in the setup register (Table 9) configures the MX7705 for unipolar or bipolar transfer functions. Figures 4 and 5 illustrate the unipolar and bipolar trans­fer functions, respectively.
In unipolar mode, the digital output code is straight binary. When AIN+ = AIN-, the outputs are at zero scale, which is the lower endpoint of the transfer func­tion. The full-scale endpoint is given by AIN+ - AIN- = V
REF
/ GAIN, where V
REF
= V
REF+
- V
REF-
.
In bipolar mode, the digital output code is in offset binary. Positive full scale is given by AIN+ - AIN- = +V
REF
/ GAIN and negative full scale is given by AIN+ -
AIN- = -V
REF
/ GAIN. When AIN+ = AIN-, the outputs are at zero scale, which is the midpoint of the bipolar transfer function.
When the MX7705 is in buffered mode, the absolute and common-mode analog input voltage ranges reduce to between (GND + 50mV) and (VDD- 1.5V). The differential input voltage range is not affected in buffered mode.
MX7705
16-Bit, Low-Power, 2-Channel,
Sigma-Delta ADC
______________________________________________________________________________________ 19
Table 5. Input Sampling Capacitor vs. Gain
Figure 4. MX7705 Unipolar Transfer Function
Figure 5. MX7705 Bipolar Transfer Function
GAIN INPUT SAMPLING CAPACITOR (C
SAMP
) (pF)
1 3.75
2 7.5
415
8–128 30
1
LSB
V
GAIN
REF
65 536
(, )
=
1
V
GAIN
REF
65 536
(, )
2
LSB
V
/ GAIN
REF
1111 1111 1111 1111
1111 1111 1111 1110
1111 1111 1111 1101
1111 1111 1111 1100
BINARY OUTPUT CODE
0000 0000 0000 0011
0000 0000 0000 0010
0000 0000 0000 0001
0000 0000 0000 0000
1 LSB =
(GAIN) (65,536)
0123
FULL-SCALE TRANSITION
V
REF
DIFFERENTIAL INPUT VOLTAGE (LSB)
/ GAIN
REF
V
65,53565,533
V
1111 1111 1111 1111
1111 1111 1111 1110
1111 1111 1111 1101
1000 0000 0000 0001
1000 0000 0000 0000
0111 1111 1111 1111
BINARY OUTPUT CODE
0000 0000 0000 0010
0000 0000 0000 0001
0000 0000 0000 0000
1 LSB =
-32,768 -32,766
/ GAIN V
REF
V
REF
(GAIN) (65,536)
x 2
0+1-1
DIFFERENTIAL INPUT VOLTAGE (LSB)
REF
/ GAIN
+32,765
+32,767
/ GAINV
REF
V
/ GAIN
REF
MX7705
Modulator
The MX7705 performs analog-to-digital conversions using a single-bit, 2nd-order, switched-capacitor, sigma-delta modulator. The sigma-delta modulation converts the input signal into a digital pulse train whose average duty cycle represents the digitized signal infor­mation. A single comparator within the modulator quan­tizes the input signal at a much higher sample rate than the bandwidth of the input.
The MX7705 modulator provides 2nd-order frequency shaping of the quantization noise resulting from the sin­gle-bit quantizer. The modulator is fully differential for maximum signal-to-noise ratio and minimum suscepti­bility to power-supply and common-mode noise. A sin­gle-bit data stream is then presented to the digital filter for processing to remove the frequency-shaped quanti­zation noise.
The modulator sampling frequency is f
CLKIN
/ 128,
regardless of gain, where f
CLKIN
(CLKDIV = 0) is the
frequency of the signal at CLKIN.
Digital Filtering
The MX7705 contains an on-chip, digital lowpass filter that processes the 1-bit data stream from the modulator using a SINC3(sinx/x)3response. The SINC3filter has a settling time of three output data periods.
Filter Characteristics
Figure 6 shows the filter frequency response. The SINC3characteristic -3dB cutoff frequency is 0.262 times the first-notch frequency. This results in a cutoff frequency of 15.72Hz for a first filter-notch frequency of 60Hz (output data rate of 60Hz). The response shown in Figure 5 is repeated at either side of the digital filter’s sample frequency, fM(fM= 19.2kHz for 60Hz output data rate), and at either side of the related harmonics (2fM, 3fM, etc.).
The output data rate for the digital filter corresponds with the positioning of the first notch of the filters frequency response. Therefore, for the plot in Figure 6, where the first notch of the filter is 60Hz, the output data rate is 60Hz. The notches of the SINC
3
filter are repeated at multiples of the first notch frequency. The SINC3filter provides an attenua­tion of better than 100dB at these notches.
Determine the cutoff frequency of the digital filter by load­ing the appropriate values into the CLK, FS0, and FS1 bits in the clock register (Table 13). Programming a differ­ent cutoff frequency with FS0 and FS1 changes the fre­quency of the notches, but it does not alter the profile of the frequency response.
For step changes at the input, allow a settling time before valid data is read. The settling time depends on the output data rate chosen for the filter. The worst­case settling time of a SINC3filter for a full-scale step input is four times the output data period. By synchro­nizing the step input using FSYNC, the settling time reduces to three times the output data period. If FSYNC is high during the step input, the filter settles in three times the output data period after FSYNC falls low.
Analog Filtering
The digital filter does not provide any rejection close to the harmonics of the modulator sample frequency. Due to the high oversampling ratio of the MX7705, these bands occupy only a small fraction of the spectrum and most broadband noise is filtered. The analog filtering requirements in front of the MX7705 are reduced com­pared to a conventional converter with no on-chip filtering. In addition, the devices provide excellent common-mode rejection of 90db to reduce the common-mode noise sus­ceptibility.
Additional filtering prior to the MX7705 eliminates unwanted frequencies the digital filter does not reject. Use additional filtering to ensure that differential noise signals outside the frequency band of interest do not saturate the analog modulator.
If passive components are in the path of the analog inputs when the device is in unbuffered mode, ensure the source impedance is low enough (Figure 2) not to introduce gain errors in the system. This significantly limits the amount of passive anti-aliasing filtering that can be applied in front of the MX7705 in unbuffered mode. In buffered mode, large source impedance causes a small DC-offset error, which can be removed by calibration.
16-Bit, Low-Power, 2-Channel, Sigma-Delta ADC
20 ______________________________________________________________________________________
Figure 6. Frequency Response of the SINC3Filter (Notch at 60Hz)
0
-20
-40
-60
-80
GAIN (dB)
-100
-120
-140
-160 0 40608020 100 120 140 160 180 200
f
CLKIN
CLK = 1 FS1 = 0 FS0 = 1 f
N
FREQUENCY (Hz)
= 60Hz
= 2.4576MHz
External Oscillator
The oscillator requires time to stabilize when enabled. Startup time for the oscillator depends on supply voltage, temperature, load capacitances, and center frequency. Depending on the load capacitance, a 1MΩ feedback resistor across the crystal can reduce the startup time (Figure 7). The MX7705 was tested with an ECS-24-32-1 (2.4576MHz crystal) and an ECS-49-20-1 (4.9152MHz crystal) (see the Typical Operating Characteristics). In power-down mode, the supply current with the external oscillator enabled is typically 67µA with a 3V supply and 227µA with a 5V supply.
Serial-Digital Interface
The MX7705 interface is fully compatible with SPI-, QSPI-, and MICROWIRE-standard serial interfaces. The serial interface provides access to seven on-chip registers. The registers are 8, 16, and 24 bits in size.
Drive CS low to transfer data in and out of the MX7705. Clock in data at DIN on the rising edge of SCLK. Data at DOUT changes on the falling edge of SCLK and is valid on the rising edge of SCLK. DIN and DOUT are trans­ferred MSB first. Drive CS high to force DOUT high impedance and cause the MX7705 to ignore any signals on SCLK and DIN. Connect CS low for 3-wire operation. Figures 8 and 9 show the timings for write and read operations, respectively.
On-Chip Registers
The MX7705 contains seven internal registers (Figure 10), which are accessed by the serial interface. These regis­ters control the various functions of the device and allow the results to be read. Table 7 lists the address, power-on default value, and size of each register.
The first of these registers is the communications register. The 8-bit communications register controls the acquisition channel selection, whether the next data transfer is a read or write operation, and which register is to be accessed. The second register is the 8-bit setup register, which con-
trols calibration modes, gain setting, unipolar/bipolar inputs, and buffered/unbuffered modes. The third register is the 8-bit clock register, which sets the digital filter char­acteristics and the clock control bits. The fourth register is the 16-bit data register, which holds the output result. The 24-bit offset and gain registers store the calibration coeffi­cients for the MX7705. The 8-bit test register is used for factory testing only.
The default state of the MX7705 is to wait for a write to the communications register. Any write or read opera­tion on the MX7705 is a two-step process. First, a com­mand byte is written to the communications register. This command selects the input channel, the desired register for the next read or write operation, and whether the next operation is a read or a write. The sec­ond step is to read from or write to the selected regis­ter. At the end of the data-transfer cycle, the device returns to the default state. See the Performing a Conversion section for examples.
If the serial communication is lost, write 32 ones to the serial interface to return the MX7705 to the default state. The registers are not reset after this operation.
MX7705
16-Bit, Low-Power, 2-Channel,
Sigma-Delta ADC
______________________________________________________________________________________ 21
Figure 7. Using a Crystal or Ceramic Oscillator
Figure 8. Write Timing Diagram
Figure 9. Read Timing Diagram
CRYSTAL OR
CERAMIC
RESONATOR
C
L
C
L
OPTIONAL
CLKIN
MX7705
CLKOUT
1M
CS
t
6
SCLK
t
2
t
10
t
9
DIN
MSB LSB
DRDY
t
8
t
6
t
7
SCLK
DOUT
t
1
CS
t
2
t
3
t
4
t
5
MSB LSB
MX7705
Communications Register
The byte-wide communications register is bidirectional so it can be written and read. The byte written to the communications register indicates the next read or write operation on the selected register, the power-down mode, and the analog input channel (Table 6). The DRDY bit indicates the conversion status.
0/DRDY: (Default = 0) Communication-Start/Data-Ready Bit. Write a 0 to the 0/DRDY bit to start a write operation to the communications register. If 0/DRDY = 1, then the device waits until a 0 is written to 0/DRDY before continu­ing to load the remaining bits. For a read operation, the 0/DRDY bit shows the status of the conversion. The DRDY bit returns a 0 if the conversion is complete and the data is ready. DRDY returns a 1 if the new data has been read and the next conversion is not yet complete. It has the same value as the DRDY output pin.
RS2, RS1, RS0: (Default = 0, 0, 0) Register-Select Bits. RS0, RS1, and RS2 select the next register to be accessed as shown in Table 7.
R/W: (Default = 0) Read-/Write-Select Bit. Use this bit to select if the next register access is a read or a write operation. Set R/W = 0 to select a write operation or set R/W = 1 for a read operation on the selected register.
PD: (Default = 0) Power-Down Control Bit. Set PD = 1 to initiate power-down mode. Set PD = 0 to take the device out of power-down mode. If CLKDIS = 0, CLKOUT remains active during power-down mode to provide a clock source for other devices in the system.
CH0, CH1: (Default = 0, 0) Channel-Select Bit. Write to the CH0 and CH1 bits to select the conversion channel or to access the calibration data shown in Table 8. The cali­bration coefficients of a particular channel are stored in one of the three offset and gain-register pairs in Table 8. Set CH1 = 1 and CH0 = 0 to evaluate the noise perfor­mance of the part without external noise sources. In this noise evaluation mode, connect AIN1- to an external volt­age within the allowable common-mode range.
Setup Register
The byte-wide setup register is bidirectional, so it can be written and read. The byte written to the setup regis­ter sets the calibration modes, PGA gain, unipolar/bipo­lar mode, buffer enable, and conversion start (Table 9).
MD1, MD0: (Default = 0, 0) Mode-Select Bits. See Table 10 for normal operating mode, self-calibration, zero-scale calibration, or full-scale calibration-mode selection.
G2, G1, G0: (Default = 0, 0, 0) Gain-Selection Bits. See Table 11 for PGA gain settings.
B/U: (Default = 0) Bipolar/Unipolar Mode Selection. Set B/U = 0 to select bipolar mode. Set B/U = 1 to select unipolar mode.
BUF: (Default = 0) Buffer-Enable Bit. For unbuffered mode, disable the internal buffer of the MX7705 to reduce power consumption by writing a 0 to the BUF bit. Write a 1 to this bit to enable the buffer. Use the internal buffer when acquiring high source-impedance input signals.
FSYNC: (Default = 1) Filter-Synchronization/ Conversion-Start Bit. Set FSYNC = 0 to begin calibration or conversion. The MX7705 performs free-running con­versions while FSYNC = 0. Set FSYNC = 1 to stop con­verting data and to hold the nodes of the digital filter, the filter-control logic, the calibration-control logic, and the analog modulator in a reset state. The DRDY output does not reset high if it is low (indicating that valid data has not yet been read from the data register) when FSYNC goes high. To clear the DRDY output, read the data register.
Clock Register
The byte-wide clock register is bidirectional, so it can be written and read. The byte written to the setup regis­ter sets the clock, filter first-notch frequency, and the output data rate (Table 12).
MXID: (Default = 1) Maxim-Identifier Bit. This is a read­only bit. Values written to this bit are ignored.
16-Bit, Low-Power, 2-Channel, Sigma-Delta ADC
22 ______________________________________________________________________________________
Figure 10. Register Summary
DIN
DOUT
*THE TEST REGISTER IS USED FOR FACTORY TESTING ONLY.
RS2 RS1 RS0
COMMUNICATIONS REGISTER
SETUP REGISTER (8 BITS)
CLOCK REGISTER (8 BITS)
DATA REGISTER (16 BITS)
TEST REGISTER (8 BITS)*
OFFSET REGISTER (24 BITS)
GAIN REGISTER (24 BITS)
REGISTER SELECT DECODER
ZERO: (Default = 0) Zero Bit. This is a read-only bit. Values written to this bit are ignored.
CLKDIS: (Default = 0) Clock-Disable Bit. Set CLKDIS = 1 to disable the clock when using a crystal or resonator across CLKIN and CLKOUT. Set CLKDIS = 1 to disable CLKOUT when using a CMOS clock source at CLKIN. CLKOUT is held low during clock disable to save power. Set CLKDIS = 0 to allow other devices to use the output signal on CLKOUT as a clock source and/or to enable the external oscillator.
CLKDIV: (Default = 0) Clock-Divider Control Bit. The MX7705 has an internal clock divider. Set this bit to 1 to divide the input clock by two. When this bit is set to 0, the MX7705 operates at the external oscillator frequency.
CLK: (Default = 1) Clock Bit. Set CLK = 1 for f
CLKIN
=
2.4576MHz with CLKDIV = 0, or 4.9152MHz with CLKDIV = 1.
MX7705
16-Bit, Low-Power, 2-Channel,
Sigma-Delta ADC
______________________________________________________________________________________ 23
Table 6. Communications Register
Table 7. Register Selection
Table 8. Channel Selection
*The test register is used for factory testing only.
Table 9. Setup Register
FUNCTION
Name 0/DRDY RS2 RS1 RS0 R/W PD CH1 CH0
Defaults 0 0 0 0 0 0 0 0
RS2 RS1 RS0 REGISTER POWER-ON RESET STATUS REGISTER SIZE (BITS)
0 0 0 Communications Register 0x00 8
0 0 1 Setup Register 0x01 8
0 1 0 Clock Register 0x05 8
0 1 1 Data Register N/A 16
1 0 0 Test Register* N/A 8
1 0 1 No Operation ——
1 1 0 Offset Register 0x1F 40 00 24
1 1 1 Gain Register 0x57 61 AB 24
CH1 CH0 AIN+ AIN-
FIRST BIT (MSB) (LSB)
COMMUNICATION
START/DATA READY
0 0 AIN1+ AIN1- 0
0 1 AIN2+ AIN2- 1
1 0 AIN1- AIN1- 0
1 1 AIN1- AIN2- 2
REGISTER SELECT
READ/WRITE
SELECT
POWER-DOWN
MODE
CHANNEL SELECT
OFFSET/GAIN
REGISTER PAIR
FUNCTION MODE CONTROL PGA GAIN CONTROL
Name MD1 MD0 G2 G1 G0 B/U BUF FSYNC
Defaults 0 0 0 0 0 0 0 1
FIRST BIT (MSB) (LSB)
BIPOLAR/UNIPOLAR
MODE
BUFFER ENABLE FSYNC
MX7705
Set CLK = 0 for optimal performance if the external clock frequency is 1MHz with CLKDIV = 0 or 2MHz with CLKDIV = 1.
FS1, FS0: (Default = 0, 1) Filter-Selection Bits. These bits determine the output data rate and the digital-filter cutoff frequency. See Table 13 for FS1 and FS0 settings. Recalibrate when the filter characteristics are changed.
Data Register
The data register is a 16-bit register that can be read and written. Figure 9 shows how to read conversion results using the data register. A write to the data regis­ter is not required, but if the data register is written, the device does not return to its normal state of waiting for a write to the communications register until all 16 bits have been written. The 16-bit data word written to the data register is ignored.
The data from the data register is read through DOUT. DOUT changes on the falling edge of SCLK and is valid on the rising edge of SCLK. The data register format is 16-bit straight binary for unipolar mode with zero scale equal to 0x0000, and offset binary for bipolar mode with zero scale equal to 0x1000.
16-Bit, Low-Power, 2-Channel, Sigma-Delta ADC
24 ______________________________________________________________________________________
Table 10. Operating-Mode Selection
Table 11. PGA Gain Selection
MD1 MD0 OPERATING MODE
0 0 Normal Mode. Use this mode to perform normal conversions on the selected analog input channel.
Self-Calibration Mode. This mode performs self-calibration on the selected channel determined from CH0 and CH1 selection bits in the communications register (Table 6). Upon completion of self-calibration, the device returns to normal mode with MD0, MD1 returning to 0, 0. The DRDY output bit goes high when self-calibration is
01
10
11
requested and returns low when the calibration is complete and a new data word is in the data register. Self­calibration performs an internal zero-scale and full-scale calibration. The analog inputs of the device are shorted together internally during zero-scale calibration and connected to an internally generated (V voltage during full-scale calibration. The offset and gain registers for the selected channel are automatically updated with the calibration data.
Zero-Scale System-Calibration Mode. This mode performs zero-scale calibration on the selected channel determined from CH0 and CH1 selection bits in the communications register (Table 6). The DRDY output bit goes high when calibration is requested and returns low when the calibration is complete and a new data word is in the data register. Performing zero-scale calibration compensates for any DC offset voltage present in the ADC and system. Ensure that the analog input voltage is stable within 1/2 LSB for the duration of the calibration sequence. The offset register for the selected channel is updated with the zero-scale system-calibration data. Upon completion of calibration, the device returns to normal mode with MD0, MD1 returning to 0, 0.
Full-Scale System-Calibration Mode. This mode performs full-scale system calibration on the selected channel determined by the CH0 and CH1 selection bits in the communications register. This calibration assigns a full­scale output code to the voltage present on the selected channel. Ensure that the analog input voltage is stable within 1/2 LSB for the duration of the calibration sequence. The DRDY output bit goes high during calibration and returns low when the calibration is complete and a new data word is in the data register. The gain register for the selected channel is updated with the full-scale system-calibration data. Upon completion of calibration, the device returns to normal mode with MD0, MD1 returning to 0, 0.
/ selected gain)
REF
G2 G1 G0 PGA GAIN
000 1
001 2
010 4
011 8
100 16
101 32
110 64
1 1 1 128
Test Register
This register is reserved for factory testing of the device. For proper operation of the MX7705, do not change this register from its default power-on reset values.
Offset and Gain-Calibration Registers
The MX7705 contains one offset register and one gain register for each input channel. Each register is 24 bits wide and can be written and read. The offset registers store the calibration coefficients resulting from a zero­scale calibration, and the gain registers store the cali­bration coefficients resulting from a full-scale calibration. The data stored in these registers are 24-bit straight binary values representing the offset or gain errors associated with the selected channel. A 24-bit read or write operation can be performed on the cali­bration registers for any selected channel. During a write operation, 24 bits of data must be written to the register, or no data is transferred.
Write to the calibration registers in normal mode only. After writing to the calibration registers, the devices implement the new offset and gain-register calibration coefficients at the beginning of a new acquisition. To ensure the results are valid, discard the first conversion result after writing to the calibration registers.
To ensure that a conversion is not made using invalid calibration data, drive FSYNC high prior to writing to the calibration registers, and then release FSYNC low to ini­tiate conversion.
Power-On Reset
At power-up, the serial-interface, logic, digital-filter, and modulator circuits are reset. The registers are set to their default values. The device returns to wait for a write to the communications register. For accurate measurements, perform calibration routines after power-up. Allow time for the external reference and oscillator to start up before starting calibration. See the Typical Operating Characteristics for typical external­oscillator startup times.
MX7705
16-Bit, Low-Power, 2-Channel,
Sigma-Delta ADC
______________________________________________________________________________________ 25
Table 13. Output Data Rate and Notch Frequency vs. Filter Select and CLKIN Frequency
*These values are given for CLKDIV = 0. External clock frequency, f
CLKIN
, can be two times the values in this column if CLKDIV = 1.
**The filter -3dB filter cutoff frequency = 0.262 x filter first-notch frequency.
Table 12. Clock Register
FUNCTION RESERVED
Name MXID ZERO ZERO CLKDIS CLKDIV CLK FS1 FS0
Defaults 1 0 0 0 0 1 0 1
FIRST BIT (MSB) (LSB)
CLKOUT
DISABLE
CLOCK
DIVIDER
CLOCK
SELECT
FILTER SELECT
CLKIN FREQUENCY
f
(MHz)*
CLKIN
1 0 0 0 20 5.24
1 0 0 1 25 6.55
1 0 1 0 100 26.20
1 0 1 1 200 52.40
2.4576 1 0 0 50 13.10
2.4576 1 0 1 60 15.70
2.4576 1 1 0 250 65.50
2.4576 1 1 1 500 131.00
CLK FS1 FS0
OUTPUT DATA RATE
(FIRST NOTCH) (Hz)
-3dB FILTER CUTOFF** (Hz)
MX7705
Reset
Drive RESET low to reset the MX7705 to power-on reset status. DRDY goes high and all communication to the MX7705 is ignored while RESET is low. Upon releasing RESET, the device must be reconfigured to begin a con­version. The device returns to waiting for a write to the communication register after a reset has been performed. Perform a calibration sequence following a reset for accurate conversions.
The MX7705 clock generator continues to run when RESET is pulled low. This allows any device running from CLKOUT to be uninterrupted when the device is in reset.
Selecting Custom Output Data Rates and
First-Notch Frequency
The recommended frequency range of the external clock is 400kHz to 5MHz. The output data rate and first notch frequency are dependent on the decimation rate of the digital filter. Table 14 shows the available decimation rates of the digital filter. The output data rate and filter first notch is calculated using the following formula:
(if CLKDIV = 1)
(if CLKDIV = 0)
Note: First-notch filter frequency = output data rate.
Performing a Conversion
At power-on reset, the MX7705 expects a write to the communications register. Writing to the communica­tions register selects the acquisition channel, read/write operation for the next register, power-down/normal mode, and address of the following register to be accessed. The MX7705 has six user-accessible regis­ters, which control the function of the device and allow the result to be read. Write to the communications reg­ister before accessing any other registers.
Writing to the clock and setup registers after configuring and initializing the host processor serial port sets up the MX7705. Use self- or system calibrations to minimize off­set and gain errors (see the Calibration section for more details). Set FSYNC = 0 to begin calibration or conver­sion. The MX7705 performs free-running acquisitions when FSYNC is low (see the Using FSYNC section). The µC can poll the DRDY bit of the communications register and read the data register when the DRDY bit returns a
0. For hardware polling, the DRDY output goes low when the new data is valid in the data register.
The data register can be read multiple times while the next conversion takes place.
The flow diagram in Figure 11 shows an example sequence required to perform a conversion on channel 1 (AIN1+ / AIN1-) after a power-on reset.
16-Bit, Low-Power, 2-Channel, Sigma-Delta ADC
26 ______________________________________________________________________________________
Table 14. Filter Select and Decimation Rate
output data rate
output data rate
128
=
f
CLKIN
Decimation Rate
×
f
CLKIN
Decimation Rate
×128
.=
×
05
CLK FS1 FS0 DECIMATION RATE
0 0 0 391
0 0 1 313
010 78
011 39
1 0 0 384
1 0 1 320
110 77
111 38
MX7705
16-Bit, Low-Power, 2-Channel,
Sigma-Delta ADC
______________________________________________________________________________________ 27
Figure 11. Sample Flow Diagram for Data Conversion
POWER-ON RESET
INITIALIZE µC/µP SERIAL
PORT
WRITE TO THE COMMUNICATIONS
REGISTER. SELECT CHANNEL 1 AND SET
NEXT OPERATION AS A WRITE TO THE
CLOCK REGISTER.
(0x20)
WRITE TO THE CLOCK REGISTER. ENABLE
EXTERNAL OSCILLATOR. SELECT
OUTPUT UPDATE RATE OF 60Hz.
WRITE TO THE COMMUNICATIONS
REGISTER. SELECT CHANNEL 1 AND SET
NEXT OPERATION AS A WRITE TO THE
WRITE TO THE SETUP REGISTER. SET
SELF-CALIBRATION MODE, GAIN TO 0, UNIPOLAR MODE, UNBUFFERED MODE. BEGIN SELF-CALIBRATION/CONVERSION
(0xA5)
SETUP REGISTER.
(0x10)
BY CLEARING FSYNC.
(0x44)
HARDWARE POLLING SOFTWARE POLLING
POLL DRDY
NOT
0 (DATA READY)
OUTPUT
WRITE TO THE COMMUNICATIONS
REGISTER. SET NEXT OPERATION AS A
READ FROM THE DATA REGISTER.
1 (DATA
READY)
(0x38)
READ THE DATA REGISTER
(16 BITS)
WRITE TO COMMUNICATIONS REGISTER.
SET NEXT OPERATION AS A READ FROM
THE COMMUNICATIONS REGISTER.
READ THE COMMUNICATIONS REGISTER
(0x08)
(8 BITS)
POLL DRDY
BIT
0 (DATA READY)
1 (DATA NOT READY)
MX7705
Using FSYNC
When FSYNC = 1, the digital filter and analog modula­tor are in a reset state, inhibiting normal operation. Set FSYNC = 0 to begin calibration or conversion.
When configured for normal operation (MD0 and MD1 set to 0), DRDY goes low 3 x 1/output data rate after FSYNC goes low to indicate that the new conversion result is ready to be read from the data register. DRDY returns high when a read operation on the data register is complete. As long as FSYNC remains low, the MX7705 performs free-running conversions with the data registers updating at the output data rate. If the valid data is not read before the next conversion result is ready, DRDY returns high for 500 x 1/f
CLKIN
before going low again to indicate a new conversion. Set FSYNC = 1 to stop converting data.
If FSYNC goes high while DRDY is low (indicating that valid data has not yet been read from the data regis­ter), DRDY does not reset high. DRDY remains low until the new data is read from the data register or until FSYNC goes low to begin a new conversion.
Table 15 provides the duration-to-mode bits and dura­tion-to-DRDY for each calibration sequence. Duration-to- mode bits provide the time required for the calibration sequence to complete (MD1 and MD0 return to 0). Duration-to-DRDY provides the time until the first conver- sion result is valid in the data register (DRDY goes low).
The pipeline delay necessary to ensure that the first conversion result is valid is tP(tP= 2000 x 1/f
CLKIN
).
When selecting self-calibration (MD1 = 0, MD0 = 1), DRDY goes low 9 x 1/output data rate + tPafter FSYNC goes low (or after a write operation to the setup register with MD1 = 0 and MD0 = 1 is performed while FSYNC is already low) to indicate new data in the data register. The pipeline delay required to ensure that the first con­version result is valid is tP(tP= 2000 x 1/f
CLKIN
).
When zero-scale or full-scale calibration is selected, DRDY goes low 4 x 1/output data rate + t
P
after FSYNC goes low (or while the zero-scale or full-scale calibra­tion command is issued when FSYNC is already low) to indicate new data in the data register (see the Calibration section).
Calibration
To compensate for errors introduced by temperature variations or system DC offsets, perform an on-chip cal­ibration. Select calibration options by writing to the MD1 and MD0 bits in the setup register (Table 9). Calibration removes gain and offset errors from the device and/or the system. Recalibrate with changes in ambient temperature, supply voltage, bipolar/unipolar mode, PGA gain, and output data rate.
The MX7705 offers two calibration modes, self-calibra­tion and system calibration. The channels of the MX7705 are independently calibrated (Table 8). The calibration coefficients resulting from a calibration sequence on a selected channel are stored in the corre­sponding offset and gain-register pair.
Self- and system calibration automatically calculate the offset and gain coefficients, which are written to the off­set and gain registers. These offset and gain coeffi­cients provide offset and gain-error correction for the specified channel.
Self-Calibration
Self-calibration compensates for offset and gain errors internal to the ADC. Prior to calibration, set the PGA gain, unipolar/bipolar mode, and input channel setting. During self-calibration, AIN+ and AIN- of the selected channel are internally shorted together. The ADC calibrates this condition as the zero-scale output level. For bipolar mode, this zero-scale point is the midscale of the bipolar transfer function.
16-Bit, Low-Power, 2-Channel, Sigma-Delta ADC
28 ______________________________________________________________________________________
Table 15. Calibration Sequences
*Duration-to-mode bits represents the completion of the calibration sequence. **Duration to DRDY represents the time at which a new conversion result is available in the data register.
CALIBRATION TYPE
(MD1, MD0)
Self-calibration (0,1)
Zero-scale system calibration (1,0)
Full-scale system calibration (1,1)
CALIBRATION SEQUENCE
Internal zero-scale calibration at selected gain + internal full-scale calibration at selected gain
Zero-scale calibration on AIN at selected gain
Full-scale calibration on AIN at selected gain
DURATION-TO-MODE
BITS*
6 x 1/output data rate 9 x 1/output data rate + t
3 x 1/output data rate 4 x 1/output data rate + t
3 x 1/output data rate 4 x 1/output data rate + t
DURATION TO DRDY**
P
P
P
Next, an internally generated voltage (V
REF
/ GAIN) is applied across AIN+ and AIN-. This condition results in the full-scale calibration.
Start self-calibration by setting MD1 = 0, MD0 = 1, and FSYNC = 0 in the setup register. Self-calibration com­pletes in 6 x 1/output data rate. The MD1 and MD0 bits both return to zero at the end of calibration. The device returns to normal acquisition mode and performs a con­version, which completes in 3 x 1/output data rate after the self-calibration sequence.
The DRDY output goes high at the start of calibration and falls low when the calibration is complete and the next conversion result is valid in the data register. The total time for self-calibration and one conversion (time until DRDY goes low) is 9 x 1/output data rate. If DRDY is low before or goes low during the calibration com­mand write to the setup register, DRDY takes up to one additional modulator cycle (128/f
CLKIN
) to return high to
indicate a calibration or conversion in progress.
System Calibration
System calibration compensates for offset and gain errors for the entire analog signal path including the ADC, signal conditioning, and signal source. System calibration is a two-step process and requires individ­ual zero-scale and full-scale calibrations on the select­ed channel at a specified PGA gain. Recalibration is recommended with changes in ambient temperature, supply voltage, bipolar/unipolar mode, PGA gain, and output data rate. Before starting calibration, set the PGA gain and the desired channel.
Set the zero-scale reference point across AIN+ and AIN-. Start the zero-scale calibration by setting MD1 = 1, MD0 = 0, and FSYNC = 0 in the setup register. When zero­scale calibration is complete (3 x 1/output data rate), MD1 and MD0 both return to zero. DRDY goes high at the start of the zero-scale system calibration and returns low when there is a valid word in the data register (4 x 1/out­put data rate). The time until DRDY goes low is com­prised of one zero-scale calibration sequence (3 x 1/output data rate) and one conversion on the AIN volt­age (1 x 1/output data rate). If DRDY is low before or goes low during the calibration command write to the setup register, DRDY takes up to one additional modula­tor cycle (128/f
CLKIN
) to return high to indicate a calibra-
tion or conversion in progress.
After performing a zero-scale calibration, connect the analog inputs to the full-scale voltage level (V
REF
/ GAIN). Perform a full-scale calibration by setting MD1 = 1 and MD0 = 1. After 3 x 1/output data rate, MD1 and MD0 both return to zero at the completion of full-scale calibration. DRDY goes high at the beginning of cali- bration and returns low after calibration is complete and new data is in the data register (4 x 1/output data rate). The time until DRDY goes low is comprised of one full-scale calibration sequence (3 x 1/output data rate) and one conversion on the AIN voltage (1 x 1/out­put data rate). If DRDY is low before or goes low during the calibration-command write to the setup register, DRDY takes up to one additional modulator cycle (128/f
CLKIN
) to return high to indicate a calibration or
conversion in progress.
In bipolar mode, the midpoint (zero scale) and positive full scale of the transfer function are used to calculate the calibration coefficients of the gain and offset registers. In unipolar mode, system calibration is performed using the two endpoints of the transfer function (Figures 4 and 5).
Power-Down Modes
The MX7705 includes a power-down mode to save power. Select power-down mode by setting PD = 1 in the communications register. The PD bit does not affect the serial interface or the status of the DRDY line. While in power-down mode, the MX7705 retains the contents of all of its registers. Placing the part in power-down mode reduces current consumption to 2µA (typ) when in external clock mode and with CLKIN connected to VDDor GND. If DRDY is high before the part enters power-down mode, then DRDY remains high until the part returns to normal operation mode and new data is available in the data register. If DRDY is low before the part enters power-down mode, indicating new data in the data register, the data register can be read during power-down mode. DRDY goes high at the end of this read operation. If the new data remains unread, DRDY stays low until the MX7705 is taken out of power-down mode and resumes data conversion. Resume normal operation by setting PD = 0. The device begins a new conversion with a result appearing in 3 x 1/output data rate + tP, where tP= 2000 x 1/f
CLKIN
, after PD is set to
0. If the clock is stopped during power-down mode, allow sufficient time for the clock to start up before resuming conversion.
If CLKDIS = 0, CLKOUT remains active during power­down mode to provide a clock source for other devices in the system.
MX7705
16-Bit, Low-Power, 2-Channel,
Sigma-Delta ADC
______________________________________________________________________________________ 29
MX7705
Applications Information
Applications Examples
Strain-Gauge Measurement
Connect the differential inputs of the MX7705 to the bridge network of the strain gauge. In Figure 12, the ana­log positive supply voltage powers the bridge network and the MX7705, along with the reference voltage in a ratiometric configuration. The on-chip PGA allows the MX7705 to handle an analog input voltage range as low as 20mV to full scale.
Temperature Measurement
Use the MX7705 for temperature measurements from a thermocouple (Figure 13). Operate the MX7705 in buffered mode to allow large decoupling capacitors at the analog inputs. The decoupling capacitors eliminate any noise pickup from the thermocouple leads. AIN1- is biased up at the reference voltage to accommodate the reduced common-mode input range in buffered mode.
Optical Isolation
For applications that require an optically isolated inter­face, see Figure 14. With 6N136-type optocouplers, maximum clock speed is 4MHz. Maximum clock speed is limited by the degree of mismatch between the indi­vidual optocouplers. Faster optocouplers allow faster signaling at a higher cost.
Layout, Grounding, Bypassing
Use PC boards with separate analog and digital ground planes. Connect the two ground planes togeth­er at the MX7705 GND. Isolate the digital supply from the analog with a low-value resistor (10) or ferrite bead when the analog and digital supplies come from the same source.
Ensure that digital return currents do not pass through the analog ground and that return-current paths are low impedance. A 5mA current flowing through a PC board ground trace impedance of only 0.05creates an error voltage of about 250µV.
Layout the PC board to ensure digital and analog sig­nal lines are kept separate. Do not run digital lines (especially the SCLK and DOUT) parallel to any analog lines. If they must cross another, do so at right angles.
Bypass VDDto the analog ground plane with a 0.1µF capacitor in parallel with a 1µF to 10µF low-ESR capac­itor. Keep capacitor leads short for best supply-noise rejection. Bypass REF+ and REF- with a 0.1µF capaci­tor to GND. Place all bypass capacitors as close to the device as possible to achieve the best decoupling.
16-Bit, Low-Power, 2-Channel, Sigma-Delta ADC
30 ______________________________________________________________________________________
Figure 12. Strain-Gauge Measurement
Figure 13. Temperature Measurement
ACTIVE GAUGE
DUMMY
GAUGE
THERMOCOUPLE
*USE A 1.225V REFERENCE FOR V
R
REF
JUNCTION
0.1µF
0.1µF
R
R
1.225V/2.5V
REFERENCE*
V
DD
10µF
0.1µF
V
DD
GND
VDD = 3V/5V*
AIN1+
AIN1-
REF+
REF-
CLKOUT
V
DD
GND
CLKIN
CS
SCLK
DIN
DOUT
DRDY
RESET
10
10
MX7705
µ
F
µ
F
CLKOUT
REF+
REF-
MX7705
AIN1+ AIN1-
µ
F
0.1
µ
F
0.1
= 3V OR A 2.5V REFERENCE FOR VDD = 5V.
DD
CLKIN
CS
SCLK
DIN
DOUT
DRDY
RESET
Definitions
Integral Nonlinearity
Integral nonlinearity (INL) is the deviation of the values on an actual transfer function from a straight line. This straight line is either a best-straight-line fit or a line drawn between the endpoints of the transfer function, once offset and gain errors have been nullified. INL for the MX7705 is measured using the endpoint method. This is the more conservative method.
Unipolar Offset Error
For an ideal converter, the first transition occurs at 0.5 LSB above zero. Offset error is the amount of deviation between the measured first transition point and the ideal point.
Bipolar Zero Error
In bipolar mode, the ideal midscale transition occurs at AIN+ - AIN- = 0. Bipolar zero error is the measured deviation from this ideal value.
Gain Error
With a full-scale analog input voltage applied to the ADC (resulting in all ones in the digital code), gain error is defined as the amount of deviation between the ideal transfer function and the measured transfer function (with the offset error or bipolar zero error removed). Gain error is usually expressed in LSB or a percent of full-scale range (%FSR).
Positive Full-Scale Error
For the ideal transfer curve, the code edge transition that causes a full-scale transition to occur is 1.5 LSB below full scale. The positive full-scale error is the dif­ference between this code transition of the ideal trans­fer function and the actual measured value at this code transition. Unlike gain error, unipolar offset error and bipolar zero error are included in the positive full-scale error measurement.
Bipolar Negative Full-Scale Error
For the ideal transfer curve, the code edge transition that causes a negative full-scale transition to occur is 0.5 LSB above negative full scale. The negative full-scale error is the difference between this code transition of the ideal transfer function and the actual measured value at this code transition.
Input Common-Mode Rejection
Input common-mode rejection (CMR) is the ability of a device to reject a signal that is common to or applied to
both input terminals. The common-mode signal can be either an AC or a DC signal or a combination of the two. CMR is often expressed in decibels. Common-mode rejection ratio (CMRR) is the ratio of the differential sig­nal gain to the common-mode signal gain.
Power-Supply Rejection Ratio
Power-supply rejection ratio (PSRR) is the ratio of the input signal change (V) to the change in the converter output (V). It is typically measured in decibels.
Chip Information
TRANSISTOR COUNT: 42,000
PROCESS: BiCMOS
MX7705
16-Bit, Low-Power, 2-Channel,
Sigma-Delta ADC
______________________________________________________________________________________ 31
Figure 14. Optically Isolated Interface
ISO
3V/5V
+V
DD
V
CC
6N136
470k
MOSI
V
CC
6N136
470k
SCK
V
CC
MISO
CS
6N136
2k
2k
470k2k
DIN
SCLK
DOUT
CS
V
MX7705
DD
MX7705
16-Bit, Low-Power, 2-Channel, Sigma-Delta ADC
32 ______________________________________________________________________________________
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information, go to www.maxim-ic.com/packages
.)
PDIPN.EPS
MX7705
16-Bit, Low-Power, 2-Channel,
Sigma-Delta ADC
______________________________________________________________________________________ 33
Package Information (continued)
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information, go to www.maxim-ic.com/packages
.)
A1
MAX
0.104
0.012
0.019
0.013
0.299
0.050
MAX
0.5120.496D
MILLIMETERS
MAX
MIN
2.35
2.65
0.10
0.30
0.35
0.49
0.23
0.32
7.40 7.60
0.40 1.27
MILLIMETERS
MAX
MIN
12.60 13.00
N MS013
20
AC
INCHES
MIN
DIM
0.093A
0.004
A1
0.014
B
0.009
C
H
E
A
C
L
e 0.050 1.27
0.291
E H 0.4190.394 10.00 10.65
0.016L
VARIATIONS:
INCHES
MINDIM
D
0.398 0.413 AA10.5010.10 16
D
0.447 0.463 AB11.7511.35 18
D 0.6140.598 15.20 2415.60 AD D 0.7130.697 17.70 2818.10 AE
0-8
SIDE VIEW
N
1
TOP VIEW
D
e
FRONT VIEW
B
SOICW.EPS
PROPRIETARY INFORMATION
TITLE:
PACKAGE OUTLINE, .300" SOIC
REV.DOCUMENT CONTROL NO.APPROVAL
21-0042
1
B
1
MX7705
16-Bit, Low-Power, 2-Channel, Sigma-Delta ADC
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
34 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2003 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
Package Information (continued)
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information, go to www.maxim-ic.com/packages
.)
TSSOP4.40mm.EPS
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