Instrumentation amplifier

An instrumentation amplifier or instrumentation amplifier ( English instrumentation amplifier or InAmp ) is a particularly precise operational amplifier circuit with very high resistance (typically 10 ⁹ bis 10 ¹² Ω ) inputs. It is also available completely as an integrated circuit with permanently installed and factory-trimmed resistors.

Instrumentation amplifiers are characterized by a particularly high common mode rejection ratio ( English CMRR common mode rejection ratio, and low input) offset voltages from.

function

In the case of integrated instrumentation amplifiers, the gain factor by means of negative feedback - unlike conventional operational amplifiers (OPV) - is predefined in certain areas by internal, trimmed resistors. With some types, the gain can also be varied within certain limits using external resistors. The instrumentation amplifier can also be set up discretely using individual operational amplifiers - however, this requires the use of very precise and balanced resistors with good thermal coupling. Because of these difficulties, instrumentation amplifiers are mostly used in the form of integrated circuits.

Because of the high common-mode rejection in relation to coupled-in stray fields, such as the power grid, instrument amplifiers are used as measurement amplifiers in measuring devices for recording EKG and EEG leads. Due to their high input impedance, they are also suitable for amplifying voltage signals from high-resistance sensors that can be measured almost without current, such as a pH meter or piezoceramic sensors. They are also used where a low offset voltage of the inputs is required, for example as a measuring amplifier on strain gauges or with thermocouples .

Circuit variants

Three OPVs

Circuit of an instrumentation amplifier with 3 OPVs

The more well-known instrumentation amplifier circuit consists of three operational amplifiers (OPV), the first two working as amplifier stages and the third being connected as a subtracter and realizing common mode rejection.

The output voltage U _a as a function of the input voltage difference of the circuit with three OPVs is:

{\ displaystyle U_ {a} = \ left (1+ {2R \ over R _ {\ mathrm {gain}}} \ right) \ cdot {R_ {3} \ over R_ {2}} \ cdot (U_ {2} -U_ {1})}

The common mode rejection depends on the precise correspondence between the ratio of the two resistors R3 and R2. These must therefore be very precise and particularly low in drift. In integrated circuits, they naturally show a similar drift because they are thermally coupled and manufactured with the same technology - an advantage of integration. The resistances in integrated instrumentation amplifier circuits are balanced at the factory, which causes the price to be higher than that of conventional operational amplifiers. Only R _{gain is} used to change the _gain , the connections of which are therefore often accessible from the outside.

The naive approach, i.e. R short-circuited and R _gain open, simply consists of a subtractor with two upstream impedance converters to increase the input resistance. In this structure, the subtracter works as an amplifier. The reason why this circuit is not used is mainly the poorer common mode rejection.

Two OPVs

Circuit of an instrumentation amplifier with 2 OPVs

Another instrumentation amplifier circuit consists of only two operational amplifiers, as shown in the adjacent figure. Functionally, it is the same as the circuit with three operational amplifiers, but places higher demands on the offset properties of the operational amplifiers used and is usually somewhat worse in practice when it comes to common-mode rejection. The advantage is the lower circuit complexity.

Even when switching with two operational amplifiers, the resistor pairs with the same designation must have the same values as possible. In this instrumentation amplifier circuit, deviations lead to the common-mode rejection deteriorating.

The gain is also set in the circuit with two OPVs with only one resistor R _gain . With balanced resistors, the output voltage U _a is given as:

{\ displaystyle U_ {a} = \ left (1+ {R_ {2} \ over R_ {1}} + {2R_ {2} \ over R _ {\ mathrm {gain}}} \ right) \ cdot (U_ { 2} -U_ {1})}

With all circuit variants, only one resistance value R _gain needs to be changed to determine the _gain , which is a particular advantage of all instrument amplifier circuits.

Examples of integrated instrumentation amplifiers

AD620 data sheet (s) (pdf; 417 kB)
AD624 data sheet (s) (pdf; 397 kB)
INA111 data sheet (en) (pdf; 769 kB)
INA114 data sheet (s)
INA116 (very high resistance ~ 3fA input bias!) Data sheet (s) (PDF; 220 kB)

literature

Walter G. Jung (Editor): OP AMP Applications . Company publication Analog Devices, 2002, ISBN 0-916550-26-5 ( online ).

Ulrich Tietze, Christoph Schenk: Semiconductor circuit technology . Springer, Berlin 2002, ISBN 3-540-42849-6 .

Analog Devices: A Designers Guide to Instrumentation Amplifiers .