AD624 is Instrumentation amplifier of Analog Devices,
designed primarily for use with low level transducers,including
load cells, strain gauges, thermocouple and pressure transducers.
Picture shows the AD624 being used as a software programmable
gain amplifier. Gain switching can be accomplished with
mechanical switches such as DIP switches or reed relays. It
should be noted that the “on” resistance of the switch in series
with the internal gain resistor becomes part of the gain equation
and will have an effect on gain accuracy.
A significant advantage in using the internal gain resistors in a
programmable gain configuration is the minimization of thermocouple
signals which are often present in multiplexed data
acquisition systems.
If the full performance of the AD624 is to be achieved, the user
must be extremely careful in designing and laying out his circuit
to minimize the remaining thermocouple signals.
AD620 useful in many bridge applications
the AD620 is especially suitable for higher resistance pressure
Transducer powered at lower voltages where small size and low
Power become more significant.
Picture shows a 3 kΩ pressure transducer bridge powered from 5 V.
In such a circuit, the bridge consumes only 1.7 mA. Adding the
AD620 and a buffered voltage divider allows the signal to be
conditioned for only 3.8 mA of total supply current. Small size and
low cost make the AD620 especially attractive for voltage output
pressure transducers.
Since it delivers low noise and drift, it will also serve applications
such as diagnostic noninvasive blood pressure measurement.
AD620 datasheet pdf
Selection of the appropriate transducer is therefore the first and most
importance step in obtaining accurate results. The equations should be
asked before a transducer can be select
- What is the physical quantity to be measured ?
For determining the type and range of the measureand.
- Which transducer principle can best be used to measure this quantity?
For determining the input and output characteristic of the
transducer be compatible with the recording or measurement system.
- What accuracy is required for this measurement?
The accuracy requirement of the total system determine the
degree to which individual factor contributing to accuracy must be
considered. Some of these factors are
1. basic electronic and mechanical characteristic of the transducer
- Type and range of measurand
- Sensitivity
- Excitation
- Mechanical and electrical connection
- Mounting provisions
- Corrosion resistance
2. Transducer accuracy , as an independent component
- Nonlinearity effect
- Hysteresis effect
- Frequency response
- Resolution
3. Transducer’s compatibility
- Zero balance provisions
- Sensitivity tolerance
- Impedance matching
- Insulation resistance