Datasheet

Data Sheet AD623
Rev. G | Page 27 of 32
Ground Returns for Input Bias Currents
Input bias currents are dc currents that must flow to bias the
input transistors of an amplifier, which 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 so that the bias current can flow. Figure 78,
Figure 79, and Figure 80 show how a bias current path can be
provided for transformer coupling, thermocouple, and
capacitive ac coupling. In dc-coupled resistive bridge
applications, providing this path is generally not necessary
because 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
proportional with the input offset voltage of the amplifier.
AD623
OUTPUT
TO POWER
SUPPLY
GROUND
REF
LOAD
+V
S
–V
S
2
1
8
3
6
5
7
4
R
G
–IN
+IN
00778-048
Figure 78. Ground Returns for Bias Currents with Transformer-Coupled
Inputs
AD623
OUTPUT
TO POWER
SUPPLY
GROUND
REF
LOAD
+V
S
–V
S
2
1
8
3
6
5
7
4
R
G
–IN
+IN
00778-049
Figure 79. Ground Returns for Bias Currents with Thermocouple Inputs
AD623
OUTPUT
TO POWER
SUPPLY
GROUND
REF
LOAD
+V
S
–V
S
2
1
8
3
6
5
7
4
R
G
–IN
+IN
100kΩ
100kΩ
00778-050
Figure 80. Ground Returns for Bias Currents with AC-Coupled Inputs
Output Buffering
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 must 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 (see Figure 81). Table 9
summarizes the performance of some buffer op amps.
5V
0.1µF
5V
0.1µF
AD623
OP113
R
G
V
IN
REFERENCE
V
OUT
00778-051
Figure 81. Output Buffering
Table 9. Buffering Options
Op Amp Description
OP113 Single-supply, high output current
OP191 Rail-to-rail input and output, low supply current
Amplifying Signals with Low Common-Mode Voltage
Because the common-mode input range of the AD623 extends
0.1 V below ground, it is possible to measure small differential
signals that have low or no common-mode component. Figure 82
shows a thermocouple application where one side of the J-type
thermocouple is grounded.
5V
0.1µF
AD623
R
G
1.02kΩ
REF
J-TYPE
THERMOCOUPLE
OUTPUT
2V
00778-053
Figure 82. Amplifying Bipolar Signals with Low Common-Mode Voltage
Over a temperature range of −200°C to +200°C, the J-type thermo-
couple delivers a voltage ranging from −7.890 mV to +10.777 m V.
A programmed gain on the AD623 of 100 (R
G
= 1.02 kΩ) and a
voltage on the REF pin of 2 V result in the output voltage ranging
from 1.110 V to 3.077 V relative to ground.