AD652
These resistors should be selected such that the following
equation holds:
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:
⎛
⎜
⎜
⎝
⎞
⎟
⎟
⎠
2RF
RG
10 V =VBRIDGE
+1
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.
+VS −5 V
RPU (max) =
5 V
−10 mA
RBRIDGE
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.
+15V
R
PU
AD652
+15V
SYNCHRONOUS
VOLTAGE-TO-
FREQUENCY
CONVERTER
R
–
BRIDGE
+
5V
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
1
5
2
REFERENCE
9
V
BRIDGE
R
F
11
7
R
G
AD625
10
V
LOGIC
15
12
16
R
F
C
INT
R
L
ONE
SHOT
2
8
FREQ
OUT
1
S1
20kΩ
CLOCK IN
1mA
–15V
Q
CK
"D"
D
AND
Q FLOP
–15V
+15V
NOTES
1. R SHOULD BE BETWEEN 10kΩ AND 20kΩ.
F
2R
F
2
F
CLOCK
10V
F
= V
BRIDGE
2. R NEEDED IF R
600Ω
+ 1
OUT
PU
BRIDGE
R
G
3. S1 IN POSITION 1 FOR UNIPOLAR SIGNALS
AND POSITION 2 FOR BIPOLAR SIGNALS.
Figure 38. Bridge Transducer Interface
Rev. C | Page 24 of 28
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