PH4705/ET4305:Instrumentation
Amp
Our sensor will be connected to some
kind of measurement system either
directly, diag. 1, or as a bridge circuit
diag 2. In either case the first stage of
the measurement system is critical. It
must not load the sensor by drawing too
much current, nor must it itself introduce
distortion or noise.
Circuits based on Operational Amplifiers
are commonly used.
PH4705/ET4305:Instrumentation
Amp
An “idea” OP Amp will have infinite input
impedance thus presenting no load on
the input. It will have zero output
impedance, so as to drive any following
circuit with ease. It will have infinite gain
so that any required gain can be set
easily with a couple of resistors. It will
have zero offset, so that a zero input
signal results in a zero output. It will have
an infinite frequency response accurately
amplifying the fastest input signal. Finally,
it will work form a wide range of power
supply voltages.
Real Op Amps easily achieve: Rin≥109Ω:
Rout≤10Ω:Gain500k:Response to
100MHz: Vs±3-18v
PH4705/ET4305:Instrumentation
R1
Amp
Practical Op Amp circuits for our
two sensor connections are a
shown.
The gain (to a first approximation )
is given by R2/R1.
To minimize the load on the
sensor R1+R2 should be as high
as practical.
Circuit 2 is preferred because it
effectively cancels noise picked
up in the bridge and input leads.
Any signal present on BOTH Op
Amp inputs cancels. The degree
of cancellation is termed Common
Mode Rejection Ratio, CMRR.
Typical values are -90dB (i.e.30k
smaller than the signal)
PH4705/ET4305:Instrumentation
Amp
This modification improves the input impedance of our circuit. The sensor is
now connected via “Unity Gain Buffer”, an Op Amp with its –ve input
connected directly to its output. This configuration has gain of 1 and an
input impedance ≥109Ω so presenting virtually no load to the sensor. Gain
and CMRR are achieved by the second part of the circuit, however, for a
good specification R1 and R2 must be matched.
PH4705/ET4305:Instrumentation
Amp
A circuit rearrangement enables gain
to be set by a single resistor – a
much more convenient arrangement.
Gain is sifted to the input OP Amps
and the ground connected R1s are
replaced by a single resistor Rgain.
The following Op Amp now has unity
gain ( all resistor the same) and is
responsible for the CMRR.
PH4705/ET4305:Instrumentation
Amp
The resultant circuit shown above is termed an Instrumentation Amplifier. Its
gain is set by the single resistor Rgain, its CMRR is very good.
Instrumentation Amps are so useful that semiconductor manufacturers
make specific ICs, e.g. Analog Devices AD620. Single resistor sets gain
between1 & 10k: wide power supply range ±2.3 to 18 v: low power 1.3mA
max supply current: high bandwidth 120kHz, fast settling: low noise: input
current 1nA max: CMRR 100dB: 8pin package.
PH4705/ET4305:Instrumentation
•
Amp
For background on non-ideal behaviour of OP Amps and the development
the instrumentation amp see:
http://www.mines.edu/Academic/courses/physics/phgn217/lab8/lect13/
Data sheet for AD620
http://www.analog.com/static/imported-files/data_sheets/AD620.pdf