Instrumentation amplifier
An instrumentation amplifier instrumentation amplifier or ( engl. instrumentation amplifier or InAmp ) is a particularly precise operational amplifier circuit with very high impedance ( typically 1 GΩ ) inputs. It is also available as complete integrated circuit with built- fix and factory- trimmed resistors.
Instrumentation amplifiers are characterized by a particularly high common-mode rejection ratio (English: CMRR - Common mode rejection ratio ) of, and low input offset voltages.
Function
The gain of using negative feedback is integrated in instrument amplifiers - unlike ordinary operational amplifiers ( OP) - internal, trimmed resistors predefined in certain areas. In some types, the gain may also be varied by external resistors within certain limits. The instrumentation amplifier can be constructed by means of individual discrete operational amplifiers - but that requires the use of very precise and concerted Reconciled resistors with good thermal coupling. Because of these difficulties instrument amplifiers are usually used in the form of integrated circuits.
Due to the high common mode rejection with respect to the coupled leakage fields, for example of the power system, the instrument amplifiers are used in instruments for recording of ECG and EEG recordings as the measured value amplifier. Because of its high input impedance, they are also suitable for the reinforcement of high-impedance voltage signals from sensors, which are almost without current to be measured, such as with a pH-meter or piezoceramic sensors. They are also used where a small offset voltage of the input is required, for example, as a measuring amplifier to strain gauges or thermocouples.
Due to their low bandwidth instrumentation amplifiers are of limited use at higher frequencies.
Circuit variations
Three OPVs
The tool -known amplifier circuit comprises three operational amplifiers (OP ), said first two operate as an amplifier stage, and the third is connected as a subtracter, and implements the common mode rejection.
The output voltage as a function of the input voltage difference in the circuit is Vout three OP:
The common-mode rejection is dependent on the precision of the correspondence relationship of the two resistors R3 and R2 to each other. These must therefore be very accurate and particularly driftarm. In integrated circuits, they naturally have a similar drift, since they are thermally coupled and were manufactured using the same technology - an advantage of the integration. The resistors in integrated instrument amplifier circuits are adjusted at the factory, which causes the over ordinary operational amplifiers higher price. To alter the gain Rgain is used exclusively, the terminals often are therefore accessible from outside.
The naiive approach, ie R is shorted and open RGain, simply consists of a subtractor with two upstream impedance converters to increase the input resistance. In this structure, the subtracter operates as an amplifier. The reason why this circuit is not used, in particular the lower common mode rejection.
Two OPVs
Another instrument amplifier circuit consists of only two operational amplifiers, as sketched in the adjacent figure. Functionally, it is equal to 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 the common-mode rejection in practice. The advantage is the reduced circuit complexity.
Also in the circuit with two operational amplifiers, the resistor pairs with the same name must have the same values as possible. For this instrument amplifier circuit variations cause worsened the common mode rejection.
The gain is set on the circuit with two operating rooms with only one resistor RGain. The output voltage Vout is given in the balanced resistors as:
In all circuit variants only one resistor value RGain must be changed to determine the gain, which is a particular advantage of all instrumentation amplifier circuits.
Examples of integrated instrumentation amplifier
- AD620 Datasheet ( s)
- INA114 data sheet (s)