AD623AR 데이터 시트보기 (PDF) - Analog Devices
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AD623AR Datasheet PDF : 24 Pages
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Ground Returns for Input Bias Currents
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
proportional with the input offset voltage of the amplifier.
+VS
–IN
2
1
7
RG
AD623
6
OUTPUT
8
3
+IN
5
REF
4
–VS
LOAD
TO POWER
SUPPLY
GROUND
Figure 49. Ground Returns for Bias Currents with Transformer-Coupled Inputs
+VS
–IN
2
1
7
RG
AD623
6
OTUPUT
8
3
+IN
5
REF
4
LOAD
–VS
TO POWER
SUPPLY
GROUND
Figure 50. Ground Returns for Bias Currents with Thermocouple Inputs
+VS
–IN
2
1
7
100kΩ
RG
8
3
+IN
100kΩ
AD623
4
–VS
6
5
REF
LOAD
OUTPUT
TO POWER
SUPPLY
GROUND
Figure 51. Ground Returns for Bias Currents with AC-Coupled Inputs
AD623
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 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
0.1µF
5V
0.1µF
VIN RG
AD623
REFERENCE
OP113
VOUT
Figure 52. Output Buffering
Table 7. Buffering Options
Op Amp Description
OP113 Single supply, high output current
OP191 Rail-to-rail input and output, low supply current
Single-Supply Data Acquisition System
Interfacing bipolar signals to single-supply ADCs presents a
challenge. The bipolar signal must be mapped into the input
range of the ADC. Figure 53 shows how this translation can be
achieved.
5V
5V
5V
0.1µF
0.1µF
±10mV
RG
1.02kΩ
AD623
AD7776
AIN
REFERENCE
REFOUT
REFIN
Figure 53. A Single-Supply Data Acquisition System
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.
Rev. D | Page 19 of 24
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