AD623
the in-amp. Resistor R1 and Capacitor C1 (and likewise, R2 and
INPUT AND OUTPUT OFFSET VOLTAGE
C2) form a low-pass RC filter that has a −3 dB bandwidth equal
to F = 1/(2 π R1C1). Using the component values shown, this
filter has a −3 dB bandwidth of approximately 40 kHz. Resistors
R1 and R2 were selected to be large enough to isolate the input of
the circuit from the capacitors, but not large enough to significantly
increase the noise of the circuit. To preserve common-mode rejection
in the amplifier’s pass band, Capacitors C1 and C2 need to be 5%
or better units, or low cost 20% units can be tested and binned
to provide closely matched devices.
The low errors of the AD623 are attributed to two sources,
input and output errors. The output error is divided by the
programmed gain when referred to the input. In practice,
the input errors dominate at high gains and the output errors
dominate at low gains. The total VOS for a given gain is calculated
as the following:
Total Error RTI = Input Error + (Output Error/G)
Total Error RTO = (Input Error × G) + Output Error
+V
S
RTI offset errors and noise voltages for different gains are
shown in Table 6.
0.33µF
0.01µF
C1
1000pF
5%
R1
4.02kΩ
1%
INPUT PROTECTION
–IN
+IN
Internal supply referenced clamping diodes allow the input,
reference, output, and gain terminals of the AD623 to safely
withstand overvoltages of 0.3 V above or below the supplies.
This is true for all gains and for power on and power off. This
last case is particularly important because the signal source
and amplifier may be powered separately.
R2
4.02kΩ
1%
C3
0.047µF
R
V
AD623
G
OUT
REFERENCE
C2
1000pF
5%
0.33µF
0.01µF
+V
S
NOTES:
1. LOCATE C1 TO C3 AS CLOSE TO THE INPUT PINS AS POSSIBLE.
If the overvoltage is expected to exceed this value, the current
through these diodes should be limited to about 10 mA using
external current limiting resistors (see Figure 44). The size of
this resistor is defined by the supply voltage and the required
overvoltage protection.
Figure 45. Circuit to Attenuate RF Interference
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 input pins of the in-amp. Any mismatch
between C1 and C2 unbalances the bridge and reduces the
common-mode rejection. C3 ensures 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 π (R1 + R2) (C3)). Using a
C3 value of 0.047 μF, the −3 dB signal bandwidth of this circuit is
approximately 400 Hz. The typical dc offset shift over frequency is
less than 1.5 μV and the circuit’s RF signal rejection is better than
71 dB. The 3 dB signal bandwidth of this circuit may be increased
to 900 Hz by reducing Resistors R1 and R2 to 2.2 kΩ. The
performance is similar to using 4 kΩ resistors, except that the
circuitry preceding the in-amp must drive a lower impedance load.
+V
S
I = 10mA MAX
LIM
V
V
AD623
OVER
OVER
R
OUTPUT
R
G
V
–V + 0.7V
S
R
OVER
LIM
R
=
LIM
10mA
–V
S
Figure 44. Input Protection
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 in Figure 45 provides good RFI
suppression without reducing performance within the pass band of
Table 6. RTI Error Sources
Maximum Total Input Offset Error (μV)
Maximum Total Input Offset Drift (μV/°C)
Total Input Referred Noise (nV/√Hz)
Gain AD623A
AD623B
600
350
200
150
125
110
105
100
AD623A
AD623B
AD623A and AD623B
1
2
5
10
20
50
100
1200
700
400
300
250
220
210
12
7
4
11
6
3
62
45
38
35
35
35
35
35
3
2
2.5
2.2
2.1
2
1.5
1.2
1.1
1
1000 200
Rev. D | Page 17 of 24
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