Analog Devices AD7861 b Datasheet

11-Bit Resolution

a

Simultaneous Sampling A/D Converter

FEATURES
11-Bit Resolution Analog-to-Digital Converter
Seven Single-Ended Analog Inputs

Four Input Channels Simultaneously Sampled
Expansion with 4 Multiplexed Inputs

Internal 2.5 V Reference

3.2 ␮s Conversion Time per Channel
User Definable Channel Sequencing
Single Supply +5 V Operation
Double Buffered Register Outputs

6.25 MHz to 12.5 MHz Operating Clock Range

APPLICATIONS
Motor Control
3-Phase Power Measurement
Cellular Phones
Data Acquisition

GENERAL DESCRIPTION

The AD7861 is a multichannel simultaneous sampling A/D
Converter (ADC) configured for the acquisition of voltage
inputs in a motor control solution or three-phase power system.

The AD7861 combined with Analog Devices’ 16-bit fixed­point digital signal processor (DSP) provides a low cost 16-bit
fixed-point microcontroller solution.

The input stage has been designed to accommodate the types of
signals frequently found in motor drives. The VIN1, VIN2, and
VIN3 channels are simultaneously sampled inputs suitable for
stator current acquisition. The AUX0–AUX3 channels are
multiplexed and are suitable for slower moving inputs such as
temperature and bus voltage of the diode rectifier output in a
motor control application.

AD7861

FUNCTIONAL BLOCK DIAGRAM

REF IN

REF OUT

VIN1
VIN2

VIN3

AUX0
AUX1

AUX2
AUX3

S0
S1

2.5V

REFERENCE

SHA

4-1

MUX

CONVST

PRODUCT HIGHLIGHTS
Simultaneous Sampling of Four Inputs

Four channel sample and hold amplifier (SHA) allows out of
phase input signals to be sampled simultaneously, preserving
the relative phase information. Sample-and-hold acquisition
time is 1.6 µs and conversion time per channel is 3.2 µs (using
a 12.5 MHz system clock).

Flexible Analog Channel Sequencing

AD7861 supports acquisition of 2, 3 or 4 channels per group.
Converted channel results are stored in registers and the data
can be read in any order. The sampling and conversion time
for two channels is 8 µs, three channels is 11.2 µs, and four
channels is 14.4 µs (using a 12.5 MHz system clock).

Single 5 V dc Operation

Low power, digital process.

11-BIT

ADC

RESET

BUSY

M0 M1

12

OUTPUT

REGISTERS

AD7861

A0
A1

D0

D11

RD
CS

CLKIN

SGND
AGND
DGND
V

DD

REV. B

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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.

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Fax: [phone] © Analog Devices, Inc., 2000

(VDD = 5 V ⴞ 5%; TA = –40ⴗC to +85ⴗC; REFIN = 2.5 V; Ext CLK @ 12.5 MHz, unless

AD7861–SPECIFICATIONS

otherwise noted)

Parameter AD7861AP Units Conditions/Comments

DC ACCURACY

Resolution 11 Bits Twos Complement Data Format
Relative Accuracy ± 2 LSB max Integral Nonlinearity
Differential Nonlinearity ± 2.5 LSB max
Bias Offset Error ± 9 LSB max Any Channel
Bias Offset Error Match 4 LSB max Between Channels
Full-Scale Error ± 13 LSB max Any Channel
Full-Scale Error Match 4 LSB max Between Channels

DYNAMIC PERFORMANCE

Signal-to-Noise Ratio (SNR) 60 dB min f
Total Harmonic Distortion (THD) –60 dB max f
Peak Harmonic or Spurious Noise –60 dB max f

= 1 kHz Sine Wave, f

IN

= 1 kHz Sine Wave, f

IN

= 1 kHz Sine Wave, f

IN

SAMPLE

SAMPLE

SAMPLE

= 75 kHz
= 75 kHz
= 75 kHz

Channel-to-Channel Isolation

M1 = 0 –58 dB max 1 kHz Sine Wave Applied to Unselected Channels
M1 = 1 –53 dB max 1 kHz Sine Wave Applied to Unselected Channels

REFERENCE

Input Voltage Range (REF IN) 2.5 V
Input Current 50 µA max
Onboard Reference Output (REF OUT) 2.5 V
Reference Tolerance ± 5%
Reference Drive Capability ± 100 µA max

SAMPLE-AND-HOLD

Acquisition Time 1.6 µs 20 CLK Cycles @ 12.5 MHz
Aperture Delay Time 200 ns max
Aperture Delay Time Match 20 ns max
Droop Rate 5 mV/ms max

LOGIC

Input High Voltage (V
Input Low Voltage (V

) 2 V min

IH

) 0.8 V max

IL

Input Leakage Current 1 µA max
Input Capacitance 20 pF typ

) 4.5 V min I

(V

OH

(V

) 0.4 V max I

OL

Current = 20 µA, VDD = 5 V

SOURCE

Current = 400 µA, VDD = 5 V

SINK

Three-State Leakage Current 1 µA max

CONVERSION RATE

Conversion Time/Channel 40 CLK Cycles

CONVST

Pulsewidth 2 CLK Cycles min

ANALOG INPUTS

Nominal Input Level 0–5 V VIN1, VIN2, VIN3, AUX0–AUX3
Input Current 100 µA
Input Capacitance 10 pF

SYSTEM CLOCK 6.25–12.5 MHz

POWER REQUIREMENTS

V

DD

I

DD

5V dc
10 mA max

– 2 –

REV. B