AMP04

REV. B

–8–

Compensating for Input and Output Errors

To achieve optimal performance, the user needs to take into

account a number of error sources found in instrumentation

amplifiers. These consist primarily of input and output offset

voltages and leakage currents.

The input and output offset voltages are independent from one

another, and must be considered separately. The input offset

component will of course be directly multiplied by the gain of

the amplifier, in contrast to the output offset voltage that is

independent of gain. Therefore, the output error is the domi-

nant factor at low gains, and the input error grows to become

the greater problem as gain is increased. The overall equation

for offset voltage error referred to the output (RTO) is:

× G) + V

OOS

voltage, and G is the programmed amplifier gain.

The change in these error voltages with temperature must also

output, is a combination of the input and output drift specifica-

tions. Again, the gain influences the input error but not the

output, and the equation is:

In some applications the user may wish to define the error con-

tribution as referred to the input, and treat it as an input error.

The relationship is:

The bias and offset currents of the input transistors also have an

impact on the overall accuracy of the input signal. The input

leakage, or bias currents of both inputs will generate an addi-

tional offset voltage when flowing through the signal source

resistance. Changes in this error component due to variations

with signal voltage and temperature can be minimized if both

input source resistances are equal, reducing the error to a

common-mode voltage which can be rejected. The difference in

bias current between the inputs, the offset current, generates a

differential error voltage across the source resistance that should

be taken into account in the user’s design.

In applications utilizing floating sources such as thermocouples,

transformers, and some photo detectors, the user must take

care to provide some current path between the high imped-

ance inputs and analog ground. The input bias currents of

the AMP04, although extremely low, will charge the stray

capacitance found in nearby circuit traces, cables, etc., and

cause the input to drift erratically or to saturate unless given a

bleed path to the analog common. Again, the use of equal resis-

tance values will create a common input error voltage that is

rejected by the amplifier.

Reference Input

ply operation it can be connected to ground to give zero volts

out with zero volts differential input. In single supply systems it

could be connected either to the negative supply or to a pseudo-

ground between the supplies. In any case, the REF input must

be driven with low impedance.

Noise Filtering

Unlike most previous instrumentation amplifiers, the output

stage’s inverting input (Pin 8) is accessible. By placing a capaci-

tor across the AMP04’s feedback path (Figure 6, Pins 6 and 8)

IN(–)

IN(+)

INPUT BUFFERS

REF

100k

1

2

11k

11k

100k

Figure 6. Noise Band Limiting

a single-pole low-pass filter is produced. The cutoff frequency

f

1

2

π (100 kΩ) C

EXT

Filtering can be applied to reduce wide band noise. Figure 7a

shows a 10 Hz low-pass filter, gain of 1000 for the AMP04.

Figures 7b and 7c illustrate the effect of filtering on noise. The

photo in Figure 7b shows the output noise before filtering. By

adding a 0.15

µF capacitor, the noise is reduced by about a

factor of 4 as shown in Figure 7c.

100k

+15V

–15V

0.15 F

Figure 7a. 10 Hz Low-Pass Filter

10

90

100

0%

5mV

10ms

Figure 7b. Unfiltered AMP04 Output