AMP01

–12–

REV. D

VOLTAGE GAIN

1M

1

10k

10k

10

100

1k

100k

1k

100

combined with the gain equation error of the AMP01 (0.6%

max for A/E grades).

All instrumentation amplifiers require attention to layout so

thermocouple effects are minimized. Thermocouples formed

between copper and dissimilar metals can easily destroy the

0.15

µV/°C. Resistors themselves can generate thermoelectric

EMF’s when mounted parallel to a thermal gradient. “Vishay”

resistors are recommended because a maximum value for ther-

moelectric generation is specified. However, where thermal

gradients are low and gain TCs of 20 ppm–50 ppm are suffi-

COMMON-MODE REJECTION

Ideally, an instrumentation amplifier responds only to the dif-

ference 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. CMR specifications are normally

measured with a full-range input voltage change and a specified

source resistance unbalance.

The current-feedback design used in the AMP01 inherently

yields high common-mode rejection. Unlike resistive feedback

designs, typified by the three-op-amp IA, the CMR is not de-

graded by small resistances in series with the reference input. A

slight, but trimmable, output offset voltage change results from

resistance in series with the reference input.

The common-mode input voltage range, CMVR, for linear

operation may be calculated from the formula:

CMVR =

± IVR –

2 G









(4)

is the maximum output signal; G is the chosen voltage gain. For

example, at +25

°C, IVR is specified as ±10.5 volt minimum

with

±15 volt supplies. Using a ±10 volt maximum swing out-

put and substituting the figures in (4) simplifies the formula to:

CMVR =

5

G









(5)

For all gains greater than or equal to 10, CMVR is

±10 volt

minimum; at gains below 10, CMVR is reduced.

ACTIVE GUARD DRIVE

Rejection of common-mode noise and line pick-up can be im-

proved by using shielded cable between the signal source and

the IA. Shielding reduces pick-up, but increases input capaci-

tance, which in turn degrades the settling-time for signal

changes. Further, any imbalance in the source resistance be-

tween the inverting and noninverting inputs, when capacitively

loaded, converts the common-mode voltage into a differential

voltage. This effect reduces the benefits of shielding. AC

common-mode rejection is improved by “bootstrapping” the

input cable capacitance to the input signal, a technique called

“guard driving.” This technique effectively reduces the input

capacitance. A single guard-driving signal is adequate at gains

above 100 and should be the average value of the two inputs.

drive the buffer amplifier (Figure 31).

GROUNDING

The majority of instruments and data acquisition systems have

separate grounds for analog and digital signals. Analog ground

may also be divided into two or more grounds which will be tied

together at one point, usually the analog power-supply ground.

In addition, the digital and analog grounds may be joined, nor-

mally at the analog ground pin on the A-to-D converter. Fol-

lowing this basic grounding practice is essential for good circuit

performance (Figure 32).

Mixing grounds causes interactions between digital circuits and

the analog signals. Since the ground returns have finite resis-

tance and inductance, hundreds of millivolts can be developed

between the system ground and the data acquisition compo-

nents. Using separate ground returns minimizes the current flow

in the sensitive analog return path to the system ground point.

Consequently, noisy ground currents from logic gates do not

interact with the analog signals.

Inevitably, two or more circuits will be joined together with their

grounds at differential potentials. In these situations, the differ-

ential input of an instrumentation amplifier, with its high CMR,

can accurately transfer analog information from one circuit to

another.

SENSE AND REFERENCE TERMINALS

The sense terminal completes the feedback path for the instru-

mentation amplifier output stage and is normally connected

directly to the output. The output signal is specified with re-

spect to the reference terminal, which is normally connected to

analog ground.