AMP01AXSLASH883C datasheet(11/22 Pages) AD

11 / 22 page

AMP01

–11–

REV. D

GAIN

The AMP01 uses two external resistors for setting voltage gain

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

tor, RS, and gain-set resistor, RG, are related by the formula:

G = 20

× R

S/RG, where G is the selected voltage gain (refer to

Figure 29).

REFERENCE

OUTPUT

V+

V–

+IN

–IN

VOLTAGE GAIN, G =

(

)

SENSE

AMP01

Figure 29. Basic AMP01 Connections for Gains

0.1 to 10,000

The magnitude of RS affects linearity and output referred errors.

Circuit performance is characterized using RS = 10 k

Ω when

operating on

±15 volt supplies and driving a ±10 volt output. R

may be reduced to 5 k

Ω in many applications particularly when

operating on

±5 volt supplies or if the output voltage swing is

limited to

±5 volts. Bandwidth is improved with R

S = 5 k

Ω and

this also increases common-mode rejection by approximately

6 dB at low gain. Lowering the value below 5 k

Ω can cause

instability in some circuit configurations and usually has no

advantage. High voltage gains between two and ten thousand

would require very low values of RG. For RS = 10 k

Ω and

AV = 2000 we get RG = 100

Ω; this value is the practical lower

limit for RG. Below 100

Ω, mismatch of wirebond and resistor

temperature coefficients will introduce significant gain tempco

errors. Therefore, for gains above 2,000, RG should be kept

constant at 100

Ω and R

S increased. The maximum gain of

10,000 is obtained with RS set to 50 k

Ω.

Metal-film or wirewound resistors are recommended for best

results. The absolute values and TCs are not too important,

only the ratiometric parameters.

AC amplifiers require good gain stability with temperature and

time, but dc performance is unimportant. Therefore, low cost

metal-film types with TCs of 50 ppm/

°C are usually adequate

for RS and RG. Realizing the full potential of the AMP01’s offset

voltage and gain stability requires precision metal-film or wire-

wound resistors. Achieving a 15 ppm/

°C gain tempco at all gains

requires RS and RG temperature coefficient matching to

5 ppm/

°C or better.

INPUT AND OUTPUT OFFSET VOLTAGES

Instrumentation amplifiers have independent offset voltages

associated with the input and output stages. While the initial

offsets may be adjusted to zero, temperature variations will

cause shifts in offsets. Systems with auto-zero can correct for

offset errors, so initial adjustment would be unnecessary. How-

ever, many high-gain applications don’t have auto zero. For

these applications, both offsets can be nulled, which has mini-

mal effect on TCVIOS and TCVOOS

The input offset component is directly multiplied by the ampli-

fier gain, whereas output offset is independent of gain. There-

fore, 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) + V

OOS

(1)

where VIOS and VOOS are the input and output offset voltage

specifications and 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) + TCV

OOS

(2)

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

the output offset voltage specification. Frequently, the amplifier

drift is referred back to the input (RTI), which is then equiva-

lent to an input signal change;

TCVOS (RTI) = TCVIOS

TCV OOS

(3)

For example, the maximum input-referred drift of an AMP01 EX

set to G = 1000 becomes;

TCVOS (RTI ) = 0.3

µV/°C +

100

1000

µVC

= 0.4

µV/°C max

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 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, should be grounded

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

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