AMP04

REV. A

–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 in-

dependent of gain. Therefore, the output error is the dominant

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:

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 out-

put, and the equation is:

× G) + TCV

OOS

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 re-

sistance. 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 cur-

rent between the inputs, the offset current, generates a differen-

tial 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 impedance in-

puts 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 resistance 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)

2

3

8

1

6

5

IN(–)

IN(+)

INPUT BUFFERS

R

GAIN

100k

REF

100k

11k

11k

R

GAIN

C

EXT

ƒ

1

2

π (100k) C

EXT

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. Fig-

ures 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.

7

1

6

5

4

3

2

8

+15V

–15V

100

0.15

µF

Figure 7a. 10 Hz Low-Pass Filter

10

90

100

0%

5mV

10ms

Figure 7b. Unfiltered AMP04 Output