AMP01EX datasheet(19/30 Pages) AD

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AMP01

Data Sheet

Rev. E | Page 18 of 29

THEORY OF OPERATION

INPUT AND OUTPUT OFFSET VOLTAGES

Instrumentation amplifiers have independent offset voltages

associated with the input and output stages. Still, temperature

variations cause offset shifts regardless of initial zero adjustments.

Systems with auto-zero correct for offset errors, rendering initial

adjustment unnecessary. However, many high gain applications

do not have auto-zero. For such applications, both offsets can be

nulled, which has minimal effect on TCVIOS and TCVOOS.

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, whereas at high gain,

input offset errors dominate. The overall offset voltage, VOS,

referred to the output (RTO) is calculated as follows:

VOS (RTO) = (VIOS × G) + VOOS

(1)

where:

VIOS is the input offset voltage specification.

VOOS is the output offset voltage specification.

G is the amplifier gain.

Input offset nulling alone is recommended with amplifiers

having fixed gain above 50. Output offset nulling alone is

recommended when gain is fixed at 50 or below.

In applications requiring both initial offsets to be nulled, the

input offset is nulled first by short circuiting RG, then the output

offset is nulled with the short removed.

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

(2)

where:

TCVIOS is the input offset voltage drift.

TCVOOS is the output offset voltage specification.

Frequently, the amplifier drift is referred back to the input

(RTI), which is then equivalent to an input signal change:

TCV

RTI

TCV

OOS

IOS



)

(

(3)

For example, the maximum input referred drift of an AMP01EX

set to G = 1000 becomes,

max

1000

100

)

(



















RTI

TCV

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 instrumentation

amplifier (IA) minimizes 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 a nontrimmable error. The magnitude 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, must be grounded close

to the signal source for best common-mode rejection.

GAIN

The AMP01 uses two external resistors for setting voltage gain

over the range of 0.1 to 10,000. The magnitudes of the scale

resistor, RS, and the gain set resistor, RG, are related by the

formula G = 20 × RS/RG, where G is the selected voltage gain

(see Figure 32).

REFERENCE

OUTPUT

V+

V–

+IN

–IN

VOLTAGE GAIN, G =

SENSE

AMP01

Figure 32. Basic AMP01 Connections for Gains of 0.1 to 10,000

The magnitude of RS affects linearity and output referred errors.

Circuit performance is characterized using RS = 10 kΩ when

operating on ±15 V supplies and driving a ±10 V output. RS can

be reduced to 5 kΩ in many applications, particularly when

operating on ±5 V supplies, or if the output voltage swing is

limited to ±5 V. Bandwidth is improved with RS = 5 kΩ,

increasing the common-mode rejection by approximately 6 dB

at low gain. Reducing the value below 5 kΩ can cause instability

in some circuit configurations and usually has no advantage.

High voltage gains between 2 and 10,000 require very low

values of RG. For RS = 10 kΩ and AV = 2000, RG = 100 Ω; this

value is the practical lower limit for RG. Below 100 Ω, mismatch

of wire bond and resistor temperature coefficients (TCs)

introduce significant gain TC errors. Therefore, for gains above

2000, RG must be kept constant at 100 Ω and RS increased. The

maximum gain of 10,000 is obtained with RS set to 50 kΩ.

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