Datasheet
Table Of Contents
- Features
- Applications
- General Description
- Functional Block Diagram
- Revision History
- Specifications
- Absolute Maximum Ratings
- Pin Configuration and Function Descriptions
- Typical Performance Characteristics
- Theory of Operation
- Applications Information
- Evaluation Board
- Outline Dimensions
Data Sheet AD623
Rev. G | Page 25 of 32
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 overvoltage protection is true at all gain settings and when
cycling power on and off. Overvoltage protection is particularly
important because the signal source and amplifier can be
powered separately.
If the overvoltage exceeds this value, limit the current through
these diodes to about 10 mA using external current-limiting
resistors (see Figure 73). The size of this resistor is defined by
the supply voltage and the required overvoltage protection.
R
G
V
OVER
V
OVER
AD623
OUTPUT
+V
S
–V
S
R
LIM
R
LIM
I = 10mA MAX
R
LIM
=
V
OVER
–V
S
+ 0.7V
10mA
00778-043
Figure 73. Input Protection
RF INTERFERENCE
All instrumentation amplifiers can rectify high frequency out-
of-band signals. When rectified, these signals appear as dc
offset errors at the output. The circuit in Figure 74 provides RFI
suppression without reducing performance within the pass band of
the instrumentation amplifier. Resistor 1 (R1) and Capacitor 1
(C1), and likewise, Resistor 2 (R2) and Capacitor 2 (C2), form a
low-pass resistor capacitor (RC) filter that has a −3 dB bandwidth
equal to f = 1/(2 π R1C1). Using the component values shown in
Figure 74, this filter has a −3 dB bandwidth of approximately 40
kHz. The R1 and R2 resistors were chosen 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 pass band of the
amplifier, the C1 and C2 capacitors must be ±5% tolerance, or
low cost 20% capacitors can be tested and binned to provide
closely matched devices.
00778-044
R
G
–IN
+IN
AD623
V
OUT
R1
4.02kΩ
1%
R2
4.02kΩ
1%
REFERENCE
+V
S
0.01µF
0.33µF
+V
S
0.01µF0.33µF
C1
1000pF
5%
C3
0.047µF
C2
1000pF
5%
NOTES:
1. LOCATE C1 TO C3 AS CLOSE TO THE INPUT PINS AS POSSIBLE.
Figure 74. Circuit to Attenuate RF Interference
C3 is needed to maintain common-mode rejection at low
frequencies. R1 and R2, as well as C1 and C2, form a bridge
circuit whose output appears across the input pins of the
instrumentation amplifier. 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 instrumentation amplifier inputs) and are
not applied differentially. This second low-pass network, R1 + R2
and C3, has a −3 dB frequency equal to 1/(2π(R1 + R2)(C3)).
Using a C3 value of 0.047 µF, t he −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 RF signal rejection of the
circuit is greater than 71 dB. The 3 dB signal bandwidth of this
circuit can be increased to 900 Hz by reducing R1 and R2 to
2.2 kΩ. The performance is similar to using 4 kΩ resistors,
except that the circuitry preceding the instrumentation amplifier
must drive a lower impedance load.
Table 8. RTI Error Sources
Maximum Total Input Offset Error (µV) Maximum Total Input Offset Drift (µV/°C) Total Input Referred Noise (nV/√Hz)
Gain
AD623ANZ,
AD623ARZ
AD623BNZ,
AD623BRZ
AD623ANZ,
AD623ARZ
AD623BNZ,
AD623BRZ
AD623ANZ,
AD623ARZ
AD623BNZ,
AD623BRZ
1 1200 600 12 11 62 62
2 700 350 7 6 45 45
5 400 200 4 3 38 38
10 300 150 3 2 35 35
20
250
125
2.5
1.5
35
35
50 220 110 2.2 1.2 35 35
100 210 105 2.1 1.1 35 35
1000 200 100 2 1 35 35