AD623
+V
+V
S
S
+2.5V TO +6V
+3V TO +12V
10F
10F
0.1F
0.1F
R
R
G
G
V
R
V
R
G
OUTPUT
REF
OUTPUT
REF
V
V
OUT
IN
G
IN
OUT
R
R
G
G
REF (INPUT)
REF (INPUT)
10F
0.1F
–2.5V TO –6V
–V
S
a. Dual Supply
b. Single Supply
Figure 41. Basic Connections
Table I. Required Values of Gain Resistors INPUT PROTECTION
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 off. This last
case is particularly important since the signal source and ampli-
fier may be powered separately.
Desired
Gain
1% Std Table
Value of RG, ⍀
Calculated Gain
Using 1% Resistors
2
5
10
20
33
40
50
65
100
200
500
1000
100 k
24.9 k
11 k
5.23 k
3.09 k
2.55 k
2.05 k
1.58 k
1.02 k
499
2
5.02
10.09
20.12
33.36
40.21
49.78
64.29
99.04
201.4
501
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. This is shown in Figure 42.
The size of this resistor is defined by the supply voltage and the
required overvoltage protection.
+V
S
200
100
1 = 10mA MAX
R
LIM
1001
V
OVER
R
AD623
R
OUTPUT
G
INPUT AND OUTPUT OFFSET VOLTAGE
R
LIM
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 domi-
nate at low gains. The total VOS for a given gain is calculated as:
V
V
؊V +0.7V
OVER
S
OVER
=
LIM
10mA
؊V
S
Figure 42. Input Protection
RF INTERFERENCE
Total Error RTI = Input Error + (Output Error/G)
Total Error RTO = (Input Error × G) + Output Error
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 43 provides good RFI
suppression without reducing performance within the in amps
pass band. 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 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
circuit’s input from the capacitors, but not large enough to
significantly increase the circuit’s noise. 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.
RTI offset errors and noise voltages for different gains are shown
below in Table II.
Table II. RTI Error Sources
Max
Max
Total Input
Offset Error
Total Input
Offset Drift
Total Input
Referred Noise
(nV/√Hz)
Gain V
V
V/؇C
V/؇C
AD623A AD623B AD623A AD623B AD623A & AD623B
1
2
5
10
20
50
1200
700
400
300
250
220
600
350
200
150
125
110
105
100
12
7
4
11
6
3
62
45
38
35
35
35
35
35
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
mismatch between C1 and C2 will unbalance the bridge and
reduce common-mode rejection. C3 ensures that any RF signals
3
2
2.5
2.2
2.1
2
1.5
1.2
1.1
1
100 210
1000 200
–12–
REV. C
ADI [ ADI ]
