AD623
Ground Returns for Input Bias Currents
Output Buffering
Input bias currents are those dc currents that must flow to bias
the input transistors of an amplifier. These are usually transistor
base currents. When amplifying floating input sources, such as
transformers or ac-coupled sources, there must be a direct dc
path into each input in order that the bias current can flow.
Figure 49, Figure 50, and Figure 51 show how a bias current
path can be provided for the cases of transformer coupling,
thermocouple, and capacitive ac coupling. In dc-coupled resistive
bridge applications, providing this path is generally not necessary
as the bias current simply flows from the bridge supply through
the bridge into the amplifier. However, if the impedances that
the two inputs see are large and differ by a large amount (>10 kΩ),
the offset current of the input stage causes dc errors
The AD623 is designed to drive loads of 10 kΩ or greater. If
the load is less than this value, the output of the AD623 should
be buffered with a precision single-supply op amp, such as the
OP113. This op amp can swing from 0 V to 4 V on its output
while driving a load as small as 600 Ω. Table 7 summarizes the
performance of some buffer op amps.
5V
5V
0.1µF
0.1µF
V
R
G
AD623
IN
OP113
V
OUT
REFERENCE
proportional with the input offset voltage of the amplifier.
Figure 52. Output Buffering
+V
S
–IN
Table 7. Buffering Options
Op Amp Description
2
1
7
OP113
OP191
Single supply, high output current
Rail-to-rail input and output, low supply current
R
6
AD623
OUTPUT
G
5
8
3
REF
4
+IN
LOAD
–V
S
Single-Supply Data Acquisition System
TO POWER
SUPPLY
GROUND
Interfacing bipolar signals to single-supply ADCs presents a
Figure 49. Ground Returns for Bias Currents with Transformer-Coupled Inputs
challenge. The bipolar signal must be mapped into the input
range of the ADC. Figure 53 shows how this translation can be
achieved.
+V
7
S
–IN
5V
2
1
5V
5V
0.1µF
0.1µF
R
6
AD623
OTUPUT
G
5
8
3
REF
AD7776
A
IN
R
G
4
AD623
±10mV
+IN
1.02kΩ
LOAD
–V
S
REFERENCE
TO POWER
SUPPLY
GROUND
REFOUT
REFIN
Figure 50. Ground Returns for Bias Currents with Thermocouple Inputs
+V
7
S
Figure 53. A Single-Supply Data Acquisition System
–IN
2
1
The bridge circuit is excited by a 5 V supply. The full-scale output
voltage from the bridge ( 10 mV) therefore has a common-
mode level of 2.5 V. The AD623 removes the common-mode
component and amplifies the input signal by a factor of 100
(RGAIN = 1.02 kΩ). This results in an output signal of 1 V. To
prevent this signal from running into the ground rail of the
AD623, the voltage on the REF pin must be raised to at least
1 V. In this example, the 2 V reference voltage from the AD7776
ADC is used to bias the output voltage of the AD623 to 2 V 1 V.
This corresponds to the input range of the ADC.
R
6
AD623
OUTPUT
G
5
8
3
REF
4
+IN
100kΩ
LOAD
100kΩ
–V
S
TO POWER
SUPPLY
GROUND
Figure 51. Ground Returns for Bias Currents with AC-Coupled Inputs
Rev. D | Page 19 of 24
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