AD652 datasheet(25/29 Pages) AD

25 / 29 page

AD652

Rev. C | Page 24 of 28

BRIDGE TRANSDUCER INTERFACE

The circuit of Figure 38 illustrates a simple interface between

the AD652 and a bridge-type transducer. The AD652 is an ideal

choice because its buffered 5 V reference can be used as the

bridge excitation, thereby ratiometrically eliminating the gain

drift related errors. This reference provides a minimum of

10 mA of external current, which is adequate for bridge

resistance of 600 Ω and above. If, for example, the bridge

resistance is 120 Ω or 350 Ω, an external pull-up resistor (RPU) is

required. RPU and can be calculated using the following formula:

(max)

−

BRIDGE

An instrumentation amplifier is used to condition the bridge

signal before presenting it to the SVFC. With its high CMRR,

the AD652 minimizes common-mode errors and can be set to

arbitrary gains between 1 and 10,000 via three resistors,

simplifying the scaling for the part’s calibrated 10 V input range.

These resistors should be selected such that the following

equation holds:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

BRIDGE

where 10 kΩ ≤ RF ≤ 20 kΩ, and VBRIDGE is the maximum output

voltage of the bridge.

The bridge output may be unipolar, as is the case for most

pressure transducers, or it may be bipolar as in some strain

measurements. If the signal is unipolar, the reference input of

the AD625 (Pin 7) is simply grounded. If the bridge has a

bipolar output, however, the AD652 reference can be tied to

Pin 7, thereby, converting a 5 V signal (after gain) into a 0 V to

+10 V input for the SVFC.

ONE

SHOT

AND

"D"

FLOP

QCK

1mA

20k

Ω

AD652

SYNCHRONOUS

VOLTAGE-TO-

FREQUENCY

CONVERTER

REFERENCE

AD625

RBRIDGE

VBRIDGE

–

+15V

–15V

+15V

CLOCK IN

FREQ

OUT

+15V

VLOGIC

RPU

–15V

CINT

NOTES

1. RF SHOULD BE BETWEEN 10kΩ AND 20kΩ.

2. RPU NEEDED IF RBRIDGE 600Ω

3. S1 IN POSITION 1 FOR UNIPOLAR SIGNALS

AND POSITION 2 FOR BIPOLAR SIGNALS.

FOUT =VBRIDGE

FCLOCK2

10V

+ 1

2RF

Figure 38. Bridge Transducer Interface

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