Application of instrument amplifier in sensor signal conditioning circuit

The meter is a high gain, dc vortex amplifier with differential input, single end output, high input impedance and high common mode rejection ratio.Differential amplifiers and instrument amplifiers use basically the same basic components (operational amplifiers), which differ greatly from standard operational amplifiers in performance.

The standard operational amplifier is a single-terminal device whose transmission function is mainly determined by the feedback network.However, differential amplifiers and instrument amplifiers can amplify very weak differential signals with common mode signal, so they have very high common mode rejection ratio (CMR).They usually do not require an external feedback network.

The instrument amplifier is a closed-loop gain unit with differential input and its output as a single output relative to the reference end.The input impedance is symmetrical and has a large value (usually 109 or more).Unlike closed-loop gain operational amplifiers, which are determined by external resistors connected between the reverse input and output ends, instrument amplifiers use an internal feedback resistance network isolated from the signal input end.With input signals added to the two differential input terminals, the gain is set by the user either internally or via an internal or external gain resistor that is also isolated from the signal input terminal.Typical instrument amplifiers have gain Settings ranging from 1 to 1000.

The characteristics of instrument amplifiers are as follows:

(1) high common mode rejection ratio

Common mode rejection ratio (CMRR) is the ratio of difference mode gain (Ad) to common mode gain (Ac), that is, CMRR=20lg|Ad/Ac|dB;Instrument amplifiers have a high common mode rejection ratio, with typical CMRR values above 70-100db.

(2) high input impedance

Required instrumentation amplifier must have extremely high input impedance, the instrumentation amplifier with phase and reverse phase at the input impedance is very high and very balance each other, the typical value is 109 a 1012 Ω low noise due to the instrumentation amplifier must be able to deal with very low input voltage, so the instrumentation amplifier cannot add their own noise to the signal, under the condition of 1 KHZ, equivalent to the requirements of input noise at the input is less than 10 nv/Hz.

(3) low linear error

Input misalignment and proportional error can be corrected by external adjustment, but linear error is the inherent defect of the device, which cannot be eliminated by external adjustment.A high quality instrumentation amplifier typically has a linear error of 0.01%, some even less than 0.0001%.

(4) low misaligned voltage and misaligned voltage drift

The misaligned drift of instrument amplifiers is also composed of input and output components, the typical values of input and output misaligned voltages are 10 mu V and 2mV respectively.

(5) error of low input bias current and misalignment current

The base current of the bipolar input operational amplifier, the grid current of the FET input operational amplifier, the bias current flowing through the unbalanced signal source resistance will produce a misalignment error.The typical bias current value of bipolar input instrument amplifier is 1nA - 50pA.The typical bias current of the FET input instrument amplifier at room temperature is 50pA.

(6) ample bandwidth

Instrument amplifiers provide adequate bandwidth for specific applications, typically ranging from 50kHz to 4MHz for small unit-gain signals.The unique feature of an instrumentation amplifier with a "check" end and a "reference" end is that it has a "check" end and a "reference" end, allowing remote detection of the output voltage while minimizing the impact of internal resistance voltage drops and ground voltage drops (IR).

In order to work effectively, the instrument amplifier is required not only to amplify the microvolt signal, but also to suppress the common mode signal of its input.This requires a large common mode rejection (CMR) in an instrument amplifier: a typical CMR value of 70-l00db.When the gain is increased, the CMR is usually improved.