ANALOG DEVICES AD7765 Service Manual

24-Bit, 156 kSPS, 112 dB Sigma-Delta ADC

REFGND

V

REF

+

BUF

V

OUT

A–

V

OUT

A+ V

IN

+ VIN– MCLK GND

OVERRANGE

DEC_RATE

AVDD1
AVDD2
AVDD3
AVDD4
DV

DD

R

BIAS

SYNC

RESET/PWRDWN

VINA+

V

IN

A–

FSO SCO SDI SDO FSI

DIFF

MULTIBIT

Σ-Δ

MODULATOR

RECONSTRUCTION

DECIMATION

FIR FILTER ENGINE

AD7765

INTERFACE L OGIC AND

OFFSET AND GAIN

CORRECTION RE GISTERS

06519-001

with On-Chip Buffers and Serial Interface

FEATURES

High performance, 24-bit Σ-Δ ADC
115 dB dynamic range at 78 kHz output data rate
112 dB dynamic range at 156 kHz output data rate
156 kHz maximum fully filtered output word rate
Pin-selectable oversampling rate (128× and 256×)
Low power mode
Flexible SPI
Fully differential modulator input
On-chip differential amplifier for signal buffering
On-chip reference buffer
Full band low-pass finite impulse response (FIR) filter
Overrange alert pin
Digital gain correction registers
Power-down mode
Synchronization of multiple devices via
Daisy chaining

APPLICATIONS

Data acquisition systems
Vibration analysis
Instrumentation

GENERAL DESCRIPTION

The AD7765 is a high performance, 24-bit sigma-delta (Σ-Δ)
analog-to-digital converter (ADC). It combines wide input
bandwidth, high speed, and performance of 112 dB dynamic
range at a 156 kHz output data rate. With excellent dc
specifications, the converter is ideal for high speed data
acquisition of ac signals where dc data is also required.

Using the AD7765 eases the front-end antialias filtering
requirements, simplifying the design process significantly. The
AD7765 offers pin-selectable decimation rates of 128× and
256×. Other features include an integrated buffer to drive the
reference, as well as a fully differential amplifier to buffer and
level shift the input to the modulator.

An overrange alert pin indicates when an input signal has
exceeded the acceptable range. The addition of internal gain
and internal overrange registers makes the AD7765 a compact,
highly integrated data acquisition device requiring minimal
peripheral components.

The AD7765 also offers a low power mode, significantly
reducing power dissipation without reducing the output data
rate or available input bandwidth.

Rev. A

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SYNC

pin

AD7765

FUNCTIONAL BLOCK DIAGRAM

Figure 1.

The differential input is sampled at up to 40 MSPS by an analog
modulator. The modulator output is processed by a series of
low-pass filters. The external clock frequency applied to the
AD7765 determines the sample rate, filter corner frequencies,
and output word rate.

The AD7765 device boasts a full band on-board FIR filter. The
full stop-band attenuation of the filter is achieved at the Nyquist
frequency. This feature offers increased protection from signals
that lie above the Nyquist frequency being aliased back into the
input signal bandwidth.

The reference voltage supplied to the AD7765 determines the
input range. With a 4 V reference, the analog input range is
±3.2768 V differential, biased around a common mode of

2.048 V. This common-mode biasing can be achieved using
the on-chip differential amplifier, further reducing the external
signal conditioning requirements.

The AD7765 is available in a 28-lead TSSOP package and is
specified over the industrial temperature range of −40°C
to +85°C.

Tabl e 1. Related Devices

Part No. Description

AD7760 2.5 MSPS, 100 dB, parallel output, on-chip buffers
AD7762 625 kSPS, 109 dB, parallel output, on-chip buffers
AD7763 625 kSPS, 109 dB, serial output, on-chip buffers
AD7764 312 kSPS, 109 dB, serial output, on-chip buffers
AD7766 128/64/32 kSPS, 8.5 mW, 109 dB SNR
AD7767 128/64/32 kSPS, 8.5 mW, 109 dB SNR

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