Datasheet AD7895BR-2, AD7895BR-10, AD7895AR-3, AD7895AR-2, AD7895AN-3 Datasheet (Analog Devices)

5 V, 12-Bit, Serial 3.8 ms

SIGNAL

SCALING*

12-BIT

ADC

OUTPUT

REGISTER

TRACK/HOLD

AD7895

V

IN

CONVST

V

DD

GND

BUSY SCLK SDATA

REF IN

*AD7895-10, AD7895-3

a

FEATURES
Fast 12-Bit ADC with 3.8 ms Conversion Time
8-Pin Mini-DlP and SOIC
Single 5 V Supply Operation
High Speed, Easy-to-Use, Serial Interface
On-Chip Track/Hold Amplifier
Selection of Input Ranges

610 V for AD7895-10

62.5 V for AD7895-3

0 V to +2.5 V for AD7895-2
High Input Impedance
Low Power: 20 mW max
14-Bit Pin Compatible Upgrade (AD7894)

GENERAL DESCRIPTION

The AD7895 is a fast 12-bit ADC that operates from a single
+5 V supply and is housed in a small 8-pin mini-DIP and 8-pin
SOIC. The part contains a 3.8 µs successive approximation A/D
converter, an on-chip track/hold amplifier, an on-chip clock and
a high speed serial interface.

Output data from the AD7895 is provided via a high speed,
serial interface port. This two-wire serial interface has a serial
clock input and a serial data output with the external serial clock
accessing the serial data from the part.

In addition to the traditional dc accuracy specifications such as
linearity and full-scale and offset errors, the AD7895 is specified
for dynamic performance parameters, including harmonic
distortion and signal-to-noise ratio.

The part accepts an analog input range of ± 10 V (AD7895-10),
±2.5 V (AD7895-3), 0 V to 2.5 V (AD7895-2) and operates
from a single +5 V supply, consuming only 20 mW max.

The AD7895 features a high sampling rate mode and, for low
power applications, a proprietary automatic power-down mode
where the part automatically goes into power down once
conversion is complete and “wakes up” before the next conver­sion cycle.

The part is available in a small, 8-pin, 0.3" wide, plastic dual-in­line package (mini-DIP) and in an 8-pin, small outline IC (SOIC).

ADC in 8-Pin Package

AD7895

FUNCTIONAL BLOCK DIAGRAM

PRODUCT HIGHLIGHTS

1. Fast, 12-Bit ADC in 8-Pin Package

The AD7895 contains a 3.8 µs ADC, a track/hold amplifier,
control logic and a high speed serial interface, all in an 8-pin
package. This offers considerable space saving over alterna­tive solutions.

2. Low Power, Single Supply Operation
The AD7895 operates from a single +5 V supply and
consumes only 20 mW. The automatic power-down mode,
where the part goes into power-down once conversion is
complete and “wakes up” before the next conversion cycle,
makes the AD7895 ideal for battery-powered or portable
applications.

3. High Speed Serial Interface
The part provides high speed serial data and serial clock lines
allowing for an easy, two-wire serial interface arrangement.

REV. 0

Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: [phone] World Wide Web Site: http://www.analog.com
Fax: [phone] © Analog Devices, Inc., 1996

(V

= +5 V, GND = 0 V, REF IN = +2.5 V.

DD

AD7895–SPECIFICATIONS

All specifications T

MIN

to T

unless otherwise noted)

MAX

Parameter A VersionslB Versions Units Test Conditions/Comments

DYNAMIC PERFORMANCE

Signal to (Noise + Distortion) Ratio

@ +25°C 70 70 dB min fIN = 50 kHz Sine Wave, f
T

to T

MIN

MAX

Total Harmonic Distortion (THD)3–78 –78 dB max fIN = 50 kHz Sine Wave, f

Peak Harmonic or Spurious Noise
Intermodulation Distortion (IMD)

2

3

= 200 kHz

SAMPLE

70 70 dB min

= 200 kHz,

3

–89 –89 dB typ fIN = 50 kHz Sine Wave, f

3

Typically –87 dB

fa = 9 kHz, fb = 9.5 kHz, f

SAMPLE

SAMPLE

SAMPLE

= 200 kHz

= 200 kHz
2nd Order Terms –87 –87 dB typ
3rd Order Terms –87 –87 dB typ

DC ACCURACY

Resolution 12 12 Bits
Minimum Resolution for which
No Missing Codes are Guaranteed 12 12 Bits
Relative Accuracy
Differential Nonlinearity
Positive Full-Scale Error

3

3

3

± 1 ±1 LSB max Typically 0.4 LSB
± 1 ±1 LSB max
± 3 ± 2 LSB max

AD7895-2

Unipolar Offset Error ±3 ±2 LSB max

AD7895-10, AD7895-3 Only

Negative Full-Scale Error

3

±3 ±2 LSB max

Bipolar Zero Error ±4 ±3 LSB max

ANALOG INPUT

AD7895-10

Input Voltage Range ±10 ±10 Volts
Input Resistance 24 24 kΩ min

AD7895-3

Input Voltage Range ±2.5 ±2.5 Volts
Input Resistance 9 9 kΩ min

AD7895-2

Input Voltage Range 0 to +2.5 0 to +2.5 Volts
Input Current 500 500 nA max

REFERENCE INPUT

REF IN Input Voltage Range 2.375/2.625 2.375/2.625 V min/V max 2.5 V ± 5%
Input Current 1 1 µA max
Input Capacitance

3

10 10 pF max

LOGIC INPUTS

Input High Voltage, V
Input Low Voltage, V
Input Current, I
Input Capacitance, C

IN

IN

INH

INL

4

2.4 2.4 V min VDD = 5 V ± 5%

0.8 0.8 V max V

= 5 V ± 5%

DD

±10 ±10 µA max VIN = 0 V to V
10 10 pF ax

DD

LOGIC OUTPUTS

Output High Voltage, V
Output Low Voltage, V

OL

OH

4.0 4.0 V min I

0.4 0.4 V max I

SOURCE

= 1.6 mA

SINK

= 200 µA

Output Coding
AD7895-10, AD7895-3 2s Complement
AD7895-2 Straight (Natural) Binary

CONVERSION RATE

Conversion Time
Mode 1 Operation 3.8 3.8 µs max
Mode 2 Operation
Track/Hold Acquisition Time

5

3

9.8 9.8 µs max

0.5 0.5 µs max

POWER REQUIREMENTS

V

DD

I

DD

+5 +5 V nom ±5% for Specified Performance
4 4 mA max Digital Inputs @ VDD, VDD = 5 V ± 5%

Power Dissipation 20 20 mW max Typically 16 mW

Power-Down Mode

IDD @ +25°C55µA max Digital Inputs @ GND, VDD = 5 V ± 5%
T

MIN

to T

MAX

10 10 µA max Digital Inputs @ GND, VDD = 5 V ± 5%

Power Dissipation @ +25°C2525 µW max

NOTES

1

Temperature ranges are as follows: A, B Versions: –40°C to +85°C.

2

Applies to Mode 1 operation. See section on “Operating Modes.”

3

See Terminology.

4

Sample tested @ +25°C to ensure compliance.

5

This 9.8 µs includes the “wake-up” time from standby. This “wake-up” time is timed from the rising edge of CONVST, whereas conversion is timed from the falling edge of CONVST, for

CONVST pulse width the conversion time is effectively the “wake-up” time plus conversion time hence 9.8 µs. This can be seen from Figure 3. Note that if the CONVST pulse width

narrow
is greater than 6 µs then the effective conversion time will increase beyond 9.8 µs.

Specifications subject to change without notice.

–2–

REV. 0

AD7895

WARNING!

ESD SENSITIVE DEVICE

+1.6V

2.0mA

2.0mA

50pF

TO

OUTPUT

PIN

1, 2

TIMING CHARACTERISTICS

Parameter A, B Versions Units Test Conditions/Comments

t

1

t

2

t

3

t

4

t

5

t

6

NOTES

1

Sample tested at +25°C to ensure compliance. All input signals are measured with tr = tf = 1 ns (10% to 90% of +5 V) and timed from a voltage level of +1.4 V.

2

The SCLK maximum frequency is 15 MHz. Care must be taken when interfacing to account for the data access time, t

processor. These two times will determine the maximum SCLK frequency that the user’s system can operate with. See “Serial Interface” section for more information.

3

Measured with the load circuit of Figure 1 and defined as the time required for an output to cross 0.8 V or 2.0 V.

4

Derived from the measured time taken by the data outputs to change 0.5 V when loaded with the circuit of Figure 1. The measured number is then extrapolated back
to remove the effects of charging or discharging the 50 pF capacitor. This means that the time, t6, quoted in the timing characteristics is the true bus relinquish time
of the part and, as such, is independent of external bus loading capacitances.

40 ns min CONVST Pulse Width

2

35

2

35

3

60
10 ns min Data Hold Time after Falling Edge of SCLK

4

50

(VDD = +5 V, GND = 0 V, REF IN = +2.5 V)

ns min SCLK High Pulse Width
ns min SCLK Low Pulse Width
ns max Data Access Time after Falling Edge of SCLK, VDD = 5 V ± 5%

ns max Bus Relinquish Time after Falling Edge of SCLK

, and the setup time required for the user's

4

ABSOLUTE MAXIMUM RATINGS*

(TA = +25°C unless otherwise noted)

VDD to GND . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3 V to +7 V

Analog Input Voltage to GND

*Stresses above those listed under “Absolute Maximum Ratings” may cause

permanent damage to the device. This is a stress rating only and functional
operation of the device at these or any other conditions above those listed in the
operational sections of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect device reliability.

AD7895-10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ±17 V

AD7895-2, AD7895-3 . . . . . . . . . . . . . . . . . . . –5 V, +10 V

Reference Input Voltage to GND . . . . –0.3 V to V

Digital Input Voltage to GND . . . . . . . –0.3 V to V

Digital Output Voltage to GND . . . . . –0.3 V to V

+ 0.3 V

DD

+ 0.3 V

DD

+ 0.3 V

DD

Operating Temperature Range

Commercial (A, B Versions) . . . . . . . . . . . –40°C to +85°C

Storage Temperature Range . . . . . . . . . . . –65°C to +150°C

Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . . +150°C

Plastic DIP Package, Power Dissipation . . . . . . . . . . 450 mW

θ

Thermal Impedance . . . . . . . . . . . . . . . . . . . . 130°C/W

JA

Lead Temperature (Soldering, 10 sec) . . . . . . . . . . +260°C

SOIC Package, Power Dissipation . . . . . . . . . . . . . . . 450 mW

θ

Thermal Impedance . . . . . . . . . . . . . . . . . . . . 170°C/W

JA

Figure 1. Load Circuit for Access Time and Bus
Relinquish Time

Lead Temperature, Soldering

Vapor Phase (60 sec) . . . . . . . . . . . . . . . . . . . . . +215°C

Infrared (15 sec) . . . . . . . . . . . . . . . . . . . . . . . . . .+220°C

ORDERING GUIDE

Model Temperature Range Linearity Error (LSB) SNR (dB) Package Option*

AD7895AN-2 –40°C to +85°C ±1 LSB 70 dB N-8
AD7895AR-2 –40°C to +85°C ±1 LSB 70 dB SO-8
AD7895BR-2 –40°C to +85°C ± 1 LSB 70 dB SO-8

AD7895AN-10 –40°C to +85°C ±1 LSB 70 dB N-8
AD7895AR-10 –40°C to +85°C ±1 LSB 70 dB SO-8
AD7895BR-10 –40°C to +85°C ±1 LSB 70 dB SO-8

AD7895AN-3 –40°C to +125°C ±1 LSB 70 dB N-8
AD7895AR-3 –40°C to +85°C ±1 LSB 70 dB SO-8

*N = Plastic DIP, SO = SOIC.

CAUTION

ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection.
Although the AD7895 features proprietary ESD protection circuitry, permanent damage may
occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD
precautions are recommended to avoid performance degradation or loss of functionality.

REV. 0

–3–

AD7895

PIN FUNCTION DESCRIPTION

Pin Pin
No. Mnemonic Description

1 REF IN Voltage Reference Input. An external reference source should be connected to this pin to provide the refer-

ence voltage for the AD7895’s conversion process. The REF IN input is buffered on chip. The nominal ref­erence voltage for correct operation of the AD7895 is +2.5 V.

2V

IN

3 GND Analog Ground. Ground reference for track/hold, comparator, digital circuitry and DAC.
4 SCLK Serial Clock Input. An external serial clock is applied to this input to obtain serial data from the AD7895.

5 SDATA Serial Data Output. Serial data from the AD7895 is provided at this output. The serial data is clocked out

6 BUSY The BUSY pin is used to indicate when the part is doing a conversion. The BUSY pin will go high on the

7

8V

CONVST Convert Start. Edge-triggered logic input. On the falling edge of this input, the track/hold goes into its hold

DD

Analog Input Channel. The analog input range is ± 10 V (AD7895-10), ±2.5 V (AD7895-3) and 0 V to
+2.5 V (AD7895-2).

A new serial data bit is clocked out on the falling edge of this serial clock. Data is guaranteed valid for 10 ns
after this falling edge so that data can be accepted on the falling edge when a fast serial clock is used. The
serial clock input should be taken low at the end of the serial data transmission.

by the falling edge of SCLK, but the data can also be read on the falling edge of SCLK. This is possible
because data bit N is valid for a specified time after the falling edge of SCLK (data hold time) (see Figure 4).
Sixteen bits of serial data are provided with four leading zeros followed by the 12 bits of conversion data.
On the sixteenth falling edge of SCLK, the SDATA line is held for the data hold time and then is disabled
(three-stated). Output data coding is 2s complement for the AD7895-10, AD7895-3 and straight binary for
the AD7895-2.

falling edge of

mode, and conversion is initiated. If
down mode. In this case, the rising edge of

CONVST and will return low when the conversion is complete.

CONVST is low at the end of conversion, the part goes into power-

CONVST “wakes up” the part.

Positive supply voltage, +5 V ± 5%.

PIN CONFIGURATION

DIP and SOIC

REF IN

V

GND

SCLK

1
2

IN

3
4

AD7895

TOP VIEW

(Not to Scale)

8
7
6
5

V

DD

CONVST

BUSY
SDATA

–4–

REV. 0

AD7895

TERMINOLOGY
Signal to (Noise + Distortion) Ratio

This is the measured ratio of signal to (noise + distortion) at the
output of the A/D converter. The signal is the rms amplitude of
the fundamental. Noise is the rms sum of all nonfundamental
signals up to half the sampling frequency (f

/2), excluding dc.

S

The ratio is dependent upon the number of quantization levels
in the digitization process; the more levels, the smaller the
quantization noise. The theoretical signal to (noise + distortion)
ratio for an ideal N-bit converter with a sine wave input is given
by:

Signal to (Noise + Distortion) = (6.02 N + 1.76) dB

Thus for a 12-bit converter, this is 74 dB.

Total Harmonic Distortion

Total harmonic distortion (THD) is the ratio of the rms sum of
harmonics to the fundamental. For the AD7895, it is defined as:

2

2

2

2

2

+V

5

6

THD (dB) =20 log

+V

+V

V

2

3

+V

4

V

1

where V1 is the rms amplitude of the fundamental, and V2, V3,
V

, V5 and V6 are the rms amplitudes of the second through the

4

sixth harmonics.

Peak Harmonic or Spurious Noise

Peak harmonic or spurious noise is defined as the ratio of the
rms value of the next largest component in the ADC output
spectrum (up to f

/2 and excluding dc) to the rms value of the

S

fundamental. Normally, the value of this specification is
determined by the largest harmonic in the spectrum, but for
parts where the harmonics are buried in the noise floor, it will
be a noise peak.

Intermodulation Distortion

With inputs consisting of sine waves at two frequencies, fa and
fb, any active device with nonlinearities will create distortion
products at sum and difference frequencies of mfa ± nfb where
m, n = 0, 1, 2, 3, etc. Intermodulation terms are those for which
neither m or n are equal to zero. For example, the second order
terms include (fa + fb) and (fa – fb), while the third order terms
include (2 fa + fb), (2 fa – fb), (fa + 2 fb) and (fa – 2 fb).

The AD7895 is tested using the CCIF standard where two
input frequencies near the top end of the input bandwidth are
used. In this case, the second and third order terms are of
different significance. The second order terms are usually
distanced in frequency from the original sine waves, while the
third order terms are usually at a frequency close to the input
frequencies. As a result, the second and third order terms are
specified separately. The calculation of the intermodulation
distortion is as per the THD specification where it is the ratio of
the rms sum of the individual distortion products to the rms
amplitude of the fundamental expressed in dBs.

Relative Accuracy

Relative accuracy or endpoint nonlinearity is the maximum
deviation from a straight line passing through the endpoints of
the ADC transfer function.

Differential Nonlinearity

This is the difference between the measured and the ideal 1 LSB
change between any two adjacent codes in the ADC.

Positive Full-Scale Error (AD7895-10)

This is the deviation of the last code transition (01 . . . 110 to
01 . . . 111) from the ideal (4 × VREF – 1 LSB) after the
Bipolar Zero Error has been adjusted out.

Positive Full-Scale Error (AD7895-3)

This is the deviation of the last code transition (01 . . . 110 to
01 . . . 111) from the ideal ( VREF – 1 LSB) after the
Bipolar Zero Error has been adjusted out.

Positive Full-Scale Error (AD7895-2)

This is the deviation of the last code transition (11 . . . 110 to
11 . . . 111) from the ideal (VREF – 1 LSB) after the Unipolar
Offset Error has been adjusted out.

Bipolar Zero Error (AD7895-10, AD7895-3)

This is the deviation of the midscale transition (all 0s to all 1s)
from the ideal 0 V (GND).

Unipolar Offset Error (AD7895-2)

This is the deviation of the first code transition (00 . . . 000 to
00 . . . 001) from the ideal 1 LSB.

Negative Full-Scale Error (AD7895-10)

This is the deviation of the first code transition (10 . . . 000 to
10 . . . 001) from the ideal (–4 × VREF + 1 LSB) after Bipolar
Zero Error has been adjusted out.

Negative Full-Scale Error (AD7895-3)

This is the deviation of the first code transition (10 . . . 000 to
10 . . . 001) from the ideal (–VREF + 1 LSB) after Bipolar Zero
Error has been adjusted out.

Track/Hold Acquisition Time

Track/Hold acquisition time is the time required for the output
of the track/hold amplifier to reach its final value, within
±1/2 LSB, after the end of conversion (the point at which the
track/hold returns to track mode). It also applies to situations
where there is a step input change on the input voltage applied
to the V

input of the AD7895. This means that the user must

IN

wait for the duration of the track/hold acquisition time after the
end of conversion or after a step input change to V

before

IN

starting another conversion to ensure that the part operates to
specification.

REV. 0

–5–

AD7895

CONVERTER DETAILS

The AD7895 is a fast, 12-bit single supply A/D converter. It
provides the user with signal scaling, track/hold, A/D converter
and serial interface logic functions on a single chip. The A/D
converter section of the AD7895 consists of a conventional
successive-approximation converter based around an R-2R
ladder structure. The signal scaling on the AD7895-10 and
AD7895-3 allows the part to handle ± 10 V and ±2.5 V input
signals, respectively, while operating from a single +5 V supply.
The AD7895-2 accepts an analog input range of 0 V to +2.5 V.
The part requires an external +2.5 V reference. The reference
input to the part is buffered on-chip.

The AD7895 has two

operating modes, the high sampling mode and the auto sleep

ance stage of the track/hold amplifier. For the AD7895-10,
R1 = 30 kΩ, R2 = 7.5 kΩ and R3 = 10 kΩ. For the AD7895-3,
R1 = R2 = 6.5 kΩ and R3 is open circuit.

For the AD7895-10 and AD7895-3, the designed code transi­tions occur on successive integer LSB values (i.e., 1 LSB, 2 LSBs,
3 LSBs . . .). Output coding is 2s complement binary with 1 LSB
= FS/4096. The ideal input/output transfer function for the
AD7895-10 and AD7895-3 is shown in Table I.

Table I. Ideal Input/Output Code Table for the AD7895-10/-3
Digital Output

Analog Input

mode, where the part automatically goes into sleep after the end of
conversion. These modes are discussed in more detail in the
“Timing and Control” section.

A major advantage of the AD7895 is that it provides all of the
above functions in an 8-pin package, either 8-pin mini-DIP or
SOIC. This offers the user considerable spacing saving advantages
over alternative solutions. The AD7895 consumes only 20 mW
maximum, making it ideal for battery-powered applications.

Conversion is initiated on the AD7895 by pulsing the
input. On the falling edge of

CONVST, the on-chip track/hold

CONVST

goes from track to hold mode, and the conversion sequence is
started. The conversion clock for the part is generated internally
using a laser-trimmed clock oscillator circuit. Conversion time
for the AD7895 is 3.8 µs in the high sampling mode (9.8 µs for
the auto sleep mode), and the track/hold acquisition time is

0.3 µs. To obtain optimum performance from the part, the read
operation should not occur during the conversion or during
300 ns prior to the next conversion. This allows the part to
operate at throughput rates up to 192 kHz and achieve data sheet
specifications.

CIRCUIT DESCRIPTION
Analog Input Section

The AD7895 is offered as three part types: the AD7895-10,
which handles a ±10 V input voltage range; the AD7895-3,

+FSR/2 – 1 LSB
+FSR/2 – 2 LSBs 011 . . . 101 to 011 . . . 110
+FSR/2 – 3 LSBs 011 . . . 100 to 011 . . . 101

GND + 1 LSB 000 . . . 000 to 000 . . . 001
GND 111 . . . 111 to 000 . . . 000
GND – 1 LSB 111 . . . 110 to 111 . . . 111

–FSR/2 + 3 LSBs 100 . . . 010 to 100 . . . 011
–FSR/2 + 2 LSBs 100 . . . 001 to 100 . . . 010
–FSR/2 + 1 LSB 100 . . . 000 to 100 . . . 001

NOTES

1

FSR is full-scale range = 20 V (AD7895-10) and = 5 V (AD7895-3)
with REF IN = +2.5 V.

2

1 LSB = FSR/4096 = 4.883 mV (AD7895-10) and 1.22 mV (AD7895-3)
with REF IN = +2.5 V.

The analog input section for the AD7895-2 contains no biasing
resistors, and the V
amplifier directly. The analog input range is 0 V to +2.5 V into
a high impedance stage with an input current of less than
500 nA. This input is benign with no dynamic charging cur­rents. Once again, the designed code transitions occur on succes­sive integer LSB values. Output coding is straight (natural) binary
with 1 LSB = FS/4096 = 2.5 V/4096 = 0.61 mV. Table II shows
the ideal input/output transfer function for the AD7895-2.

which handles input voltage range ±2.5 V; and the AD7895-2,
which handles a 0 V to +2.5 V input voltage range.

REF IN

Analog Input

+FSR – 1 LSB

TO ADC
REFERENCE

TRACK/
HOLD

CIRCUITRY

TO INTERNAL
COMPARATOR

V

AGND

IN

R1

R2

R3

AD7895-10/AD7895-3

Figure 2. AD7895-10/AD7895-3 Analog Input Structure

Figure 2 shows the analog input section for the AD7895-10 and
AD7895-3. The analog input range of the AD7895-10 is ± 10 V
into an input resistance of typically 33 kΩ. The analog input
range of the AD7895-3 is ±2.5 V into an input resistance of
typically 12 kΩ. This input is benign with no dynamic charging
currents, as the resistor stage is followed by a high input imped-

+FSR – 2 LSB 111 . . . 101 to 111 . . . 110
+FSR – 3 LSB 111 . . . 100 to 111 . . . 101

GND + 3 LSB 000 . . . 010 to 000 . . . 011
GND + 2 LSB 000 . . . 001 to 000 . . . 010
GND + 1 LSB 000 . . . 000 to 000 . . . 001

NOTES

1

FSR is full-scale range and is 2.5 V for AD7895-2 with VREF = +2.5 V.

2

1 LSB = FSR/4096 and is 0.61 mV for AD7895-2 with VREF = +2.5 V.

Track/Hold Section

The track/hold amplifier on the analog input of the AD7895
allows the ADC to accurately convert an input sine wave of full­scale amplitude to 12-bit accuracy. The input bandwidth of the
track/hold is greater than the Nyquist rate of the ADC even
when the ADC is operated at its maximum throughput rate of
192 kHz (i.e., the track/hold can handle input frequencies in
excess of 100 kHz).

l

2

pin drives the input to the track/hold

IN

Code Transition

011 . . . 110 to 011 . . . 111

Table II. Ideal Input/Output Code Table for AD7895-2

1

2

Digital Output
Code Transition

111 . . . 110 to 111 . . . 111

–6–

REV. 0

AD7895

The track/hold amplifier acquires an input signal to 12-bit
accuracy in less than 0.3 µs. The operation of the track/hold is
essentially transparent to the user. With the high sampling
operating mode, the track/hold amplifier goes from its tracking
mode to its hold mode at the start of conversion (i.e. the falling
edge of
the delay time between the external

CONVST). The aperture time for the track/hold (i.e.

CONVST signal and the

track/hold actually going into hold) is typically 15 ns. At the
end of conversion (on the falling edge of BUSY), the part returns
to its tracking mode. The acquisition time of the track/hold
amplifier begins at this point. For the auto shut down mode, the
rising edge of

CONVST wakes up the part and the track, and

hold amplifier goes from its tracking mode to its hold mode 6 µs
after the rising edge of

CONVST (provided that the CONVST

high time is less than 6 µs). Once again, the part returns to its
tracking mode at the end of conversion when the BUSY signal
goes low.

Reference Input

The reference input to the AD7895 is buffered on-chip with a
maximum reference input current of 1 µA. The part is specified
with a +2.5 V reference input voltage. Errors in the reference
source will result in gain errors in the AD7895’s transfer
function and will add to the specified full-scale errors on the
part. Suitable reference sources for the AD7895 include the
AD780 and AD680 precision +2.5 V references.

Timing and Control Section

Figure 3 shows the timing and control sequence required to
obtain optimum performance from the AD7895. In the se­quence shown, conversion is initiated on the falling edge of
CONVST, and new data from this conversion is available in
the output register of the AD7895 3.8 µs later. Once the read
operation has taken place, a further 300 ns should be allowed
before the next falling edge of

CONVST to optimize the settling
of the track/hold amplifier before the next conversion is initi­ated. With the serial clock frequency at its maximum of
15 MHz, the achievable throughput rate for the part is 3.8 µs
(conversion time) plus 1.1 µs (read time) plus 0.3 µs (acquisi-
tion time). This results in a minimum throughput time of 8.2 µs
(equivalent to a throughput rate of 192 kHz). A serial clock of
less than 15 MHz can be used, but this will in turn mean that
the throughput time will increase.

The read operation consists of sixteen serial clock pulses to the
output shift register of the AD7895. After sixteen serial clock
pulses, the shift register is reset, and the SDATA line is three-

stated. If there are more serial clock pulses after the sixteenth
clock, the shift register will be moved on past its reset state.
However, the shift register will be reset again on the falling edge
of the

CONVST signal to ensure that the part returns to a
known state every conversion cycle. As a result, a read opera­tion from the output register should not straddle across the
falling edge of

CONVST as the output shift register will be reset
in the middle of the read operation, and the data read back into
the microprocessor will appear invalid.

OPERATING MODES
Mode 1 Operation (High Sampling Performance)

The timing diagram in Figure 3 is for optimum performance in
operating Mode 1 where the falling edge of

CONVST starts
conversion and puts the Track/Hold amplifier into its hold
mode. This falling edge of CONVST also causes the BUSY
signal to go high to indicate that a conversion is taking place.
The BUSY signal goes low when the conversion is complete,
which is 3.8 µs max after the falling edge of

CONVST, and new
data from this conversion is available in the output register of
the AD7895. A read operation accesses this data. This read
operation consists of 16 clock cycles, and the length of this read
operation will depend on the serial clock frequency. For the
fastest throughput rate (with a serial clock of 15 MHz, 5 V
operation) the read operation will take 1.1 µs. The read opera-
tion must be complete at least 300 ns before the falling edge of
the next

CONVST, and this gives a total time of 5.2 µs for the

full throughput time (equivalent to 192 kHz). This mode of
operation should be used for high sampling applications.

Mode 2 Operation (Auto Sleep After Conversion)

The timing diagram in Figure 4 is for optimum performance in
operating mode 2 where the part automatically goes into sleep
mode once BUSY goes low after conversion and “wakes-up”
before the next conversion takes place. This is achieved by keep­ing

CONVST low at the end of conversion, whereas it was high
at the end of conversion for Mode 1 Operation. The rising edge
of

CONVST “wakes up” the part. This wake-up time is 6 µs at

which point the Track/Hold amplifier goes into its hold mode,
provided the

CONVST has gone low. The conversion takes

3.8 µs after this giving a total of 9.8 µs from the rising edge of
CONVST to the conversion being complete, which is indi­cated by the BUSY going low. Note that since the wake-up time
from the rising edge of

CONVST is 6 µs, when the CONVST

pulse width is greater than 6 µs, the conversion will take more

REV. 0

CONVST

BUSY

SCLK

CONVERSION IS

INITIATED AND

TRACK/HOLD GOES INTO

HOLD

t

1

t

CONVERT

= 3.8µs

CONVERSION

ENDS

3.8µs LATER

SERIAL READ

OPERATION

300ns MIN

READ OPERATION

SHOULD END 300ns

PRIOR TO NEXT

FALLING EDGE OF

CONVST

t

OUTPUT

SERIAL

SHIFT

REGISTER

IS RESET

Figure 3. Mode 1 Timing Operation Diagram for High Sampling Performance

–7–

= 40ns MIN

1

AD7895

than the 9.8 µs shown in diagram from the rising edge of
CONVST. This is because the Track/Hold amplifier goes into
its hold mode on the falling edge of

CONVST, and the conver-

sion will not be complete for a further 3.8 µs. In this case, the
BUSY will be the best indicator for when the conversion is
complete. Even though the part is in sleep mode, data can still
be read from the part. The read operation consists of 16 clock
cycles as in Mode 1 Operation. For the fastest serial clock of
15 MHz, the read operation will take 1.1 µs and this must be
complete at least 300 ns before the falling edge of the next
CONVST to allow the Track/Hold amplifier to have enough
time to settle. This mode is very useful when the part is convert­ing at a slow rate as the power consumption will be significantly
reduced from that of Mode 1 Operation.

Serial Interface

The serial interface to the AD7895 consists of just three wires: a
serial clock input (SCLK), the serial data output (SDATA) and
a conversion status output (BUSY). This allows for an easy-to­use interface to most microcontrollers, DSP processors and shift
registers.

Figure 5 shows the timing diagram for the read operation to the
AD7895. The serial clock input (SCLK) provides the clock
source for the serial interface. Serial data is clocked out from the
SDATA line on the falling edge of this clock and is valid on
both the rising and falling edges of SCLK. The advantage of
having the data valid on both the rising and falling edges of the
SCLK is that it gives the user greater flexibility in interfacing to
the part and allows a wider range of microprocessor and micro­controller interfaces to be accommodated. This also explains the
two timing figures, t
The time t

specifies how long after the falling edge of the

4

SCLK that the next data bit becomes valid, whereas the time t

and t

4

that are quoted on the diagram.

5,

5

specifies how long after the falling edge of the SCLK that the
current data bit is valid for. The first leading zero is clocked out
on the first rising edge of SCLK. Note that the first zero will be

t

1

CONVST

valid on the first falling edge of SCLK even though the data
access time is specified at 60 ns for the other bits. The reason
that the first bit will be clocked out faster than the other bits is
due to the internal architecture of the part. Sixteen clock pulses
must be provided to the part to access to full conversion result.
The AD7895 provides four leading zeros, followed by the 12-bit
conversion result starting with the MSB (DB11). The last data
bit to be clocked out on the penultimate falling clock edge is the
LSB (DB0). On the sixteenth falling edge of SCLK, the LSB
(DB0) will be valid for a specified time to allow the bit to be
read on the falling edge of the SCLK, then the SDATA line is
disabled (three-stated). After this last bit has been clocked
out, the SCLK input should return low and remain low until the
next serial data read operation. If there are extra clock pulses
after the sixteenth clock, the AD7895 will start over again with
outputting data from its output register, and the data bus will no
longer be three-stated even when the clock stops. Provided the
serial clock has stopped before the next falling edge of

CONVST,
the AD7895 will continue to operate correctly with the output
shift register being reset on the falling edge of
However, the SCLK line must be low when

CONVST.

CONVST goes low in

order to reset the output shift register correctly.
The serial clock input does not have to be continuous during the

serial read operation. The sixteen bits of data (four leading
zeros and 12 bit conversion result) can be read from the AD7895
in a number of bytes.

The AD7895 counts the serial clock edges to know which bit
from the output register should be placed on the SDATA
output. To ensure that the part does not lose synchronization,
the serial clock counter is reset on the falling edge of the
CONVST input, provided the SCLK line is low. The user
should ensure that the SCLK line remains low until the end of
the conversion. When the conversion is complete, BUSY goes
low, the output register will be loaded with the new conversion
result and can be read from with sixteen clock cycles of SCLK.

t

= 6µs

1

WAKE-UP

TIME

BUSY

300ns MIN

SCLK

t

= 9.8µs

CONVERT

PART

WAKES

UP

CONVERSION

IS INITIATED

TRACK/HOLD

GOES INTO

HOLD

CONVERSION

ENDS

9.8µs LATER

SERIAL READ

OPERATION

READ OPERATION

SHOULD END 300ns

PRIOR TO NEXT

FALLING EDGE OF

CONVST

Figure 4. Mode 2 Timing Diagram Where Automatic Sleep Function Is Initiated

t

= t3 = 35ns MIN, t4 = 60ns MAX, t5 = 10ns MIN, t6 = 50ns MAX @ 5V, A, B, VERSIONS

2

t

2

SCLK (I/P)

3-STATE

DOUT (O/P)

1 2 3 4 5 6 15 16

t

3

4 LEADING ZEROS

t

4

t

5

DB0DB10DB11

t

6

3-STATE

Figure 5. Data Read Operation

–8–

OUTPUT

SERIAL

SHIFT

REGISTER

IS RESET

REV. 0

AD7895

MICROPROCESSOR/MICROCONTROLLER INTERFACE

The AD7895 provides a three-wire serial interface that can be
used for connection to the serial ports of DSP processors and
microcontrollers. Figures 6 through 9 show the AD7895
interfaced to a number of different microcontrollers and DSP
processors. The AD7895 accepts an external serial clock, and
as a result, in all interfaces shown here, the processor/controller
is configured as the master, providing the serial clock with the
AD7895 configured as the slave in the system.

AD7895–8051 Interface

Figure 6 shows an interface between the AD7895 and the 8XL51
microcontroller. The 8XL51 is configured for its Mode 0 serial
interface mode. The diagram shows the simplest form of the
interface where the AD7895 is the only part connected to the
serial port of the 8XL51 and, therefore, no decoding of the
serial read operations is required.

P1.2

8X51/L51

INT1

P3.0

P3.1

OR

BUSY

AD7895

SDATA

SCLK

Figure 6. AD7895 to 8X51/L51 Interface

To chip select the AD7895 in systems where more than one
device is connected to the 8XL51’s serial port, a port bit
configured as an output, from one of the 8XL51’s parallel ports
can be used to gate on or off the serial clock to the AD7895. A
simple AND function on this port bit and the serial clock from
the 8XL51 will provide this function. The port bit should be
high to select the AD7895 and low when it is not selected.

The end of conversion is monitored by using the BUSY signal
that is shown in the interface diagram of Figure 6. The BUSY
line from the AD7895 is connected to the Port P1.2 of the
8XL51 so the BUSY line can be polled by the 8XL51. The BUSY
line can be connected to the INT1 line of the 8XL51 if an
interrupt driven system is preferred. These two options are
shown in the diagram.

Note also that the AD7895 outputs the MSB first during a read
operation, while the 8XL51 expects the LSB first. Therefore,
the data which is read into the serial buffer needs to be rear­ranged before the correct data format from the AD7895 appears
in the accumulator.

The serial clock rate from the 8XL51 is limited to significantly
less than the allowable input serial clock frequency with which
the AD7895 can operate. As a result, the time to read data
from the part will actually be longer than the conversion time of
the part. This means that the AD7895 cannot run at its maximum
throughput rate when used with the 8XL51.

AD7895–68HC11/L11 Interface

An interface circuit between the AD7895 and the 68HC11/L11
microcontroller is shown in Figure 7. For the interface shown,
the 68L11 SPI port is used, and the 68L11 is configured in its
single-chip mode. The 68L11 is configured in the master mode
with its CPOL bit set to a logic zero and its CPHA bit set to a
logic one. As with the previous interface, the diagram shows the
simplest form of the interface where the AD7895 is the only part
connected to the serial port of the 68L11 and, therefore, no
decoding of the serial read operations is required.

PC2 OR

68HC11/L11

MISO

IRQ

SCK

BUSY

AD7895

SCLK

SDATA

Figure 7. AD7895 to 68HC11/L11 Interface

Once again, to chip select the AD7895 in systems where more
than one device is connected to the 68HC11’s serial port, a port
bit configured as an output from one of the 68HC11’s parallel
ports can be used to gate on or off the serial clock to the
AD7895. A simple AND function on this port bit and the serial
clock from the 68L11 will provide this function. The port bit
should be high to select the AD7895 and low when it is not
selected.

The end of conversion is monitored by using the BUSY signal
that is shown in the interface diagram of Figure 7. With the
BUSY line from the AD7895 connected to the Port PC0 of the
68HC11/L11, the BUSY line can be polled by the 68HC11/L11.
The BUSY line can be connected to the

IRQ line of the
68HC11/L11 if an interrupt driven system is preferred. These
two options are shown in the diagram.

The serial clock rate from the 68HC11/L11 is limited to
significantly less than the allowable input serial clock frequency
with which the AD7895 can operate. As a result, the time to
read data from the part will actually be longer than the conver­sion time of the part. This means that the AD7895 cannot run
at its maximum throughput rate when used with the 68HC11/L11.

AD7895–ADSP-2103/5 Interface

An interface circuit between the AD7895 and the ADSP-2103/5
DSP processor is shown in Figure 8. In the interface shown, the
RFS1 output from the ADSP-2103/5s SPORT1 serial port is
used to gate the serial clock (SCLK1) of the ADSP-2103/5
before it is applied to the SCLK input of the AD7895. The
RFS1 output is configured for active high operation. The BUSY
line from the AD7895 is connected to the

IRQ2 line of the
ADSP-2103/5 so that at the end of conversion an interrupt is
generated telling the ADSP-2103/5 to initiate a read operation.
The interface ensures a noncontinuous clock for the AD7895’s
serial clock input with only sixteen serial clock pulses provided
and the serial clock line of the AD7895 remaining low between
data transfers. The SDATA line from the AD7895 is connected
to the DR1 line of the ADSP-2103/5’s serial port.

REV. 0

–9–

AD7895

957 962958 959 960 961

0

4000

3000

2000

1000

6000

5000

7000

8000

9000

IRQ2

RFS1

ADSP-2103/5

SCLK1

DR1

Figure 8. AD7896 to ADSP-2103 /5 Interface

The timing relationship between the SCLK1 and RFS1 outputs
of the ADSP-2103/5 are such that the delay between the rising
edge of the SCLK1 and the rising edge of an active high RFS1
is up to 30 ns. There is also a requirement that data must be
set up 10 ns prior to the falling edge of the SCLK1 to be read
correctly by the ADSP-2103/5. The data access time for the
AD7895 is 60 ns (5 V (A, B versions)) from the rising edge of
its SCLK input. Assuming a 10 ns propagation delay through
the external AND gate, the high time of the SCLK1 output of
the ADSP-2105 must be ≥ (30 + 60 +10 +10) ns, i.e., ≥ 110 ns.
This means that the serial clock frequency with which the
interface of Figure 8 can work is limited to 4.5 MHz. However,
there is an alternative method that allows for the ADSP-2105
SCLK1 to run at 5 MHz (the max serial clock frequency of the
SCLK1 output). The arrangement occurs when the first leading
zero of the data stream from the AD7895 cannot be guaranteed
to be clocked into the ADSP-2105 due to the combined delay of
the RFS signal and the data access time of the AD7895. In most
cases, this is acceptable because there will still be three leading
zeros followed by the 12 data bits. For the ADSP-2103, the
SCLK1 frequency will need to be limited to < 4 MHz to
account for the 100 ns data access time of the AD7895.
Another alternative scheme is to configure the ADSP-2103/5 so
that it accepts an external noncontinuous serial clock. In this
case, an external noncontinuous serial clock is provided that
drives the serial clock inputs of both the ADSP-2103/5 and the
AD7895. In this scheme, the serial clock frequency is limited to
15 MHz by the AD7895.

AD7895–DSP56002/L002 Interface

Figure 9 shows an interface circuit between the AD7895 and the
DSP56002/L002 DSP processor. The DSP56002/L002 is
configured for normal mode asynchronous operation with gated
clock. It is also set up for a 16-bit word with SCK as gated
clock output. In this mode, the DSP56002/L002 provides
sixteen serial clock pulses to the AD7895 in a serial read
operation. Because the DSP56002/L002 assumes valid data on
the first falling edge of SCK, the interface is simply two-wire as
shown in Figure 9.

MODA / IRQA

DSP56002/L002

SCK

SDR

Figure 9. AD7895 to DSP56002/L002 Interface

BUSY

SCLK

SDATA

BUSY

SCLK

SDATA

AD7895

AD7895

Because the BUSY line from the AD7895 is connected to the
MODA/

IRQA input of the DSP56002/L002, an interrupt will
be generated at the end of conversion. This ensures that the
read operation will take place after conversion is finished.

AD7895 PERFORMANCE
Linearity

The linearity of the AD7895 is determined by the on-chip
12-bit D/A converter. This is a segmented DAC that is laser
trimmed for 12-bit integral linearity and differential linearity.
Typical relative accuracy numbers for the part are ± 1/4 LSB
while the typical DNL errors are ±1/2 LSB.

Noise

In an A/D converter, noise exhibits itself as code uncertainty in
dc applications and as the noise floor (in an FFT, for example)
in ac applications. In a sampling A/D converter like the AD7895,
all information about the analog input appears in the baseband
from dc to 1/2 the sampling frequency. The input bandwidth of
the track/hold exceeds the Nyquist bandwidth and, therefore, an
antialiasing filter should be used to remove unwanted signals above
f

/2 in the input signal in applications where such signals exist.

S

Figure 10 shows a histogram plot for 8192 conversions of a dc
input using the AD7895. The analog input was set at the center
of a code transition. It can be seen that almost all the codes
appear in the one output bin, indicating very good noise
performance from the ADC.

Figure 10. Histogram of 8192 Conversions of a DC Input

In this case where the output data read for the device occurs
during conversion, this has the effect of injecting noise onto the
die while bit decisions are being made, and this increases the
noise generated by the AD7895. A histogram plot for 8192
conversions of the same dc input would show a larger spread of
codes with the rms noise for the AD7895 increasing. This effect
will vary depending on where the serial clock edges appear with
respect to the bit trials of the conversion process. It is possible
to achieve the same level of performance when reading during
conversion as when reading after conversion, depending on the
relationship of the serial clock edges to the bit trial points.

–10–

REV. 0

Dynamic Performance (Mode 1 Only)

0 1000200 400 600 800

10.0

11.4

11.2

11.0

10.8

11.8

11.6

12.0

10.6

10.4

FREQUENCY – kHz

ENOB

10.2

With a combined conversion and acquisition time of 4.1 µs, the
AD7895 is ideal for wide bandwidth signal processing applica­tions. These applications require information on the ADC’s
effect on the spectral content of the input signal. Signal to
(Noise + Distortion), Total Harmonic Distortion, Peak Har­monic or Spurious Noise, and Intermodulation Distortion are
all specified. Figure 11 shows a typical FFT plot of a 10 kHz,
0 V to +5 V input after being digitized by the AD7895 operating
at a 198.656 kHz sampling rate. The Signal to (Noise + Distor­tion) Ratio is 73.04 dB, and the Total Harmonic Distortion is
–84.91 dB.

The formula for Signal to (Noise + Distortion) Ratio (see
Terminology section) is related to the resolution or number of
bits in the converter. Rewriting the formula, below, gives a
measure of performance expressed in effective number of bits (N):

N = (SNR 1.76)/6.02

where SNR is Signal to (Noise + Distortion) Ratio.

–0
–10
–20
–30
–40
–50
–60
–70
–80
–90

–100
–110
–120

0 9.9k10k 30k 50k 70k 90k

F

= 198656

SAMPLE

= 10kHz

F

IN

SNR = –73.04dB
THD = –84.91dB

Figure 11. AD7896 FFT Plot Effective Number of Bits

The effective number of bits for a device can be calculated from
its measured Signal to (Noise + Distortion) Ratio. Figure 12
shows a typical plot of effective number of bits versus frequency
for the AD7895 from dc to f

SAMPLING

/2. The sampling frequency
is 198.656 kHz. The plot shows that the AD7895 converts an
input sine wave of 10 kHz to an effective numbers of bits of

11.84, which equates to a Signal to (Noise + Distortion) level of

73.04 dB.

AD7895

Figure 12. Effective Number of Bits vs. Frequency

Power Considerations

In the automatic power-down mode, then, the part may be
operated at a sample rate that is considerably less than
100 kHz. In this case, the power consumption will be reduced
and will depend on the sample rate. Figure 13 shows a graph
of the power consumption versus sampling rates from 100 Hz to
90 kHz in the automatic power-down mode. The conditions
are 5 V supply 25°C, serial clock frequency of 8.33 MHz, and
the data was read after conversion.

11
10

9
8
7
6
5

POWER – mW

4
3
2
1
0

0.1 9010 20 30 40
FREQUENCY – kHz

Figure 13. Power vs. Sample Rate in Auto Power-Down
Mode

50 60 70 80

REV. 0

–11–

AD7895

0.210 (5.33)
MAX

0.160 (4.06)

0.115 (2.93)

0.022 (0.558)

0.014 (0.356)

OUTLINE DIMENSIONS

Dimensions shown in inches and (mm).

Plastic DIP (N-8)

0.430 (10.92)

0.348 (8.84)

8

14

PIN 1

0.100

(2.54)

BSC

5

0.280 (7.11)

0.240 (6.10)

0.060 (1.52)

0.015 (0.38)

0.070 (1.77)

0.045 (1.15)

0.130
(3.30)
MIN

SEATING
PLANE

0.325 (8.25)

0.300 (7.62)

0.015 (0.381)

0.008 (0.204)

SOIC (SO-8)

0.1968 (5.00)

0.1890 (4.80)

0.195 (4.95)

0.115 (2.93)

C2209–12–10/96

0.2440 (6.20)

0.2284 (5.80)

0.0098 (0.25)

0.0040 (0.10)

SEATING

PLANE

85

PIN 1

0.0500
(1.27)

BSC

0.1574 (4.00)

0.1497 (3.80)

41

0.102 (2.59)

0.094 (2.39)

0.0192 (0.49)

0.0138 (0.35)

0.0098 (0.25)

0.0075 (0.19)

0.0196 (0.50)

0.0099 (0.25)

8°
0°

0.0500 (1.27)

0.0160 (0.41)

x 45°

–12–

PRINTED IN U.S.A.

REV. 0