AD627A AD [Analog Devices], AD627A Datasheet - Page 15

• Manufacturer Part Number: AD627A
• Description: Micropower, Single and Dual Supply Rail-to-Rail Instrumentation Amplifier
• Manufacturer: AD [Analog Devices]

RF INTERFERENCE
All instrumentation amplifiers can rectify high frequency out-of-
band signals. Once rectified, these signals appear as dc offset
errors at the output. The circuit of Figure 44 provides good RFI
suppression without reducing performance within the in amp’s
passband. Resistor R1 and capacitor C1 (and likewise, R2 and
C2) form a low pass RC filter that has a –3 dB BW equal to:
F = 1/(2
filter has a –3 dB bandwidth of approximately 8 kHz. Resistors
R1 and R2 were selected to be large enough to isolate the circuit’s
input from the capacitors, but not large enough to significantly
increase the circuit’s noise. To preserve common-mode rejec-
tion in the amplifier’s pass band, capacitors C1 and C2 need to
be 5% mica units, or low cost 20% units can be tested and
“binned” to provide closely matched devices.
Capacitor C3 is needed to maintain common-mode rejection at
the low frequencies. R1/R2 and C1/C2 form a bridge circuit
whose output appears across the in amp’s input pins. Any mis-
match between C1 and C2 will unbalance the bridge and reduce
common-mode rejection. C3 insures that any RF signals are
common mode (the same on both in amp inputs) and are not
applied differentially. This second low pass network, R1 + R2
and C3, has a –3 dB frequency equal to: 1/(2 (R1 + R2) (C3)).
Using a C3 value of 0.022 F as shown, the –3 dB signal BW of
this circuit is approximately 200 Hz. The typical dc offset shift
over frequency will be less than 1 mV and the circuit’s RF signal
rejection will be better than 57 dB. The 3 dB signal bandwidth
REV. A
+IN
–IN
Figure 44. Circuit to Attenuate RF Interference
THE INPUT PINS AS POSSIBLE
LOCATE C1–C3 AS CLOSE TO
R1C1). Using the component values shown, this
20k
20k
1%
1%
R1
R2
0.022 F
1000pF
1000pF
5%
5%
C1
C3
C2
Figure 43. Optimal Ground Practice in a Single Supply Environment
R
G
0.33 F
0.33 F
AD627
–V
+V
S
S
0.01 F
0.01 F
AD627
REFERENCE
0.1 F
V
OUT
–15–
V
IN
POWER SUPPLY
V
+5V
of this circuit may be increased by reducing the value of resistors
R1 and R2. The performance is similar to that using 20 k
resistors, except that the circuitry preceding the in amp must
drive a lower impedance load.
The circuit of Figure 44 should be built using a PC board with a
ground plane on both sides. All component leads should be as
short as possible. Resistors R1 and R2 can be common 1%
metal film units but capacitors C1 and C2 need to be 5%
tolerance devices to avoid degrading the circuit’s common-
mode rejection. Either the traditional 5% silver mica units or
Panasonic 2% PPS film capacitors are recommended.
APPLICATIONS CIRCUITS
A Classic Bridge Circuit
Figure 45 shows the AD627 configured to amplify the signal
from a classic resistive bridge. This circuit will work in either
dual or single supply mode. Typically the bridge will be excited
by the same voltage as is used to power the in amp. Connecting
the bottom of the bridge to the negative supply of the in amp (usu-
ally either 0, –5 V, –12 V or –15 V), sets up an input common
mode voltage that is optimally located midway between the
supply voltages. It is also appropriate to set the voltage on the
REF pin to midway between the supplies, especially if the input
signal will be bipolar. However the voltage on the REF pin can
be varied to suit the application. A good example of this is when
the REF pin is tied to the V
Converter (ADC) whose input range is (V
available output swing on the AD627 of (–V
(+V
output range divided by the input range.
DD
ADC
0.1 F
S
– 150 mV) the maximum programmable gain is simply this
AGND DGND
GND
AD7892-2
Figure 45. A Classic Bridge Circuit
V
12
DIFF
V
DD
PROCESSOR
+V
R
–V
REF
G
0.1 F
S
S
= 200k
GAIN-5
pin of an Analog-to-Digital
DGND
AD627
REF
0.1 F
0.1 F
S
+ 100 mV) to
V
AD627
IN
). With an
V
V
OUT
REF