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AMP02E

High Accuracy Instrumentation Amplifier

Analog Devices 

AMP02

APPLICATIONS INFORMATION

Input and Output Offset Voltages

Instrumentation amplifiers have independent offset voltages

associated with the input and output stages. The input offset

component is directly multiplied by the amplifier gain, whereas

output offset is independent of gain. Therefore at low gain,

output-offset errors dominate while at high gain, input-offset

errors dominate. Overall offset voltage, VOS, referred to the

output (RTO) is calculated as follows:

( ) ( ) VOS RTO = VIOS × G + VOOS

where VIOS and VOOS are the input and output offset voltage

specifications and G is the amplifier gain.

The overall offset voltage drift TCVOS, referred to the output, is

a combination of input and output drift specifications. Input

offset voltage drift is multiplied by the amplifier gain, G, and

summed with the output offset drift:

( ) ( ) TCVOS RTO = TCVIOS × G + TCVOOS

where TCVIOS is the input offset voltage drift, and TCVOOS is

the output offset voltage drift. Frequently, the amplifier drift is

referred back to the input (RTI), which is then equivalent to an

input signal change:

( ) TCVOS

RTI

= TCVIOS

+

TCVOOS

G

For example, the maximum input-referred drift of an

AMP02EP set to G = 1000 becomes:

( ) TCVOS

RTI

= 2 µV

oC + 100 µV

oC

= 2.1 µV

1000

oC

Input Bias and Offset Currents

Input transistor bias currents are additional error sources that

can degrade the input signal. Bias currents flowing through the

signal source resistance appear as an additional offset voltage.

Equal source resistance on both inputs of an IA will minimize

offset changes due to bias current variations with signal voltage

and temperature; however, the difference between the two bias

currents (the input offset current) produces an error. The mag-

nitude of the error is the offset current times the source resistance.

A current path must always be provided between the differential

inputs and analog ground to ensure correct amplifier operation.

Floating inputs such as thermocouples should be grounded

close to the signal source for best common-mode rejection.

Gain

The AMP02 only requires a single external resistor to set the

voltage gain. The voltage gain, G, is:

G = 50 kΩ +1

RG

and

50 kΩ

RG = G – 1

The voltage gain can range from 1 to 10,000. A gain set resistor is

not required for unity-gain applications. Metal-film or wirewound

resistors are recommended for best results.

The total gain accuracy of the AMP02 is determined by the

tolerance of the external gain set resistor, RG, combined with the

gain equation accuracy of the AMP02. Total gain drift combines

the mismatch of the external gain set resistor drift with that of the

internal resistors (20 ppm/°C typ). Maximum gain drift of the

AMP02 independent of the external gain set resistor is 50 ppm/°C.

All instrumentation amplifiers require attention to layout so

thermocouple effects are minimized. Thermocouples formed

between copper and dissimilar metals can easily destroy the

TCVOS performance of the AMP02, which is typically 0.5 µV/°C.

Resistors themselves can generate thermoelectric EMFs when

mounted parallel to a thermal gradient.

The AMP02 uses the triple op amp instrumentation amplifier

configuration with the input stage consisting of two transimped-

ance amplifiers followed by a unity-gain differential amplifier.

The input stage and output buffer are laser-trimmed to increase

gain accuracy. The AMP02 maintains wide bandwidth at all

gains as shown in Figure 3. For voltage gains greater than 10,

the bandwidth is over 200 kHz. At unity gain, the bandwidth of

the AMP02 exceeds 1 MHz.

80

60 G = 1000

TA = 25؇C

VS = ؎15V

G = 100

40

G = 10

20

G=1

0

–20

–40

1k

10k

100k

1M

10M

FREQUENCY – Hz

Figure 3. The AMP02 Keeps Its Bandwidth at

High Gains

Common-Mode Rejection

Ideally, an instrumentation amplifier responds only to the differ-

ence between the two input signals and rejects common-mode

voltages and noise. In practice, there is a small change in output

voltage when both inputs experience the same common-mode

voltage change; the ratio of these voltages is called the

common-mode gain. Common-mode rejection (CMR) is the

logarithm of the ratio of differential-mode gain to common-mode

gain, expressed in dB. Laser trimming is used to achieve the

high CMR of the AMP02.

–8–

REV. E

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