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AD623

Rev. D | Page 19 of 24

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.

AD623

OUTPUT

TO POWER

SUPPLY

GROUND

REF

LOAD

+VS

–VS

–IN

+IN

Figure 49. Ground Returns for Bias Currents with Transformer-Coupled Inputs

AD623

OTUPUT

TO POWER

SUPPLY

GROUND

REF

LOAD

+VS

–VS

–IN

+IN

Figure 50. Ground Returns for Bias Currents with Thermocouple Inputs

AD623

OUTPUT

TO POWER

SUPPLY

GROUND

REF

LOAD

+VS

–VS

–IN

+IN

100kΩ

Figure 51. 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 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.

0.1µF

AD623

OP113

VIN

REFERENCE

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.

0.1µF

AD623

1.02kΩ

REFERENCE

±10mV

AIN

REFOUT

REFIN

AD7776

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.

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