Hi… Can you please tell me as to what is a guard circuit used in bridges?
Mail from SN
The very high impedance amps for bridge circuits have inputs in tera ohms and bias in pico amps. The sensitive bridge circuits must be able to detect a 1 uV difference when a 100 gm weight is placed on the cantilever. So it is very sensitive.
The power lines and other logic lines close to chip high impedance pins may leak pico amps to nano amps of currents, humid-air or PCB contamination or power pins being too close, the leakage currents may show as full scale reading without input.
” The basic principle is to protect the sensitive wire with a surrounding guard that is held at the same voltage as the wire, if they are at the same voltage then there will be no current flow.”
The guard rings around these sensitive pins or shielding will act as return paths for the leaking nano amps.Even ionic discharge from a sharp pin with high voltage can imbalance bridge. As it is like an Electrometer.
Read more here. guard rings in strain gauge measurement
Strain gage transducers usually employ four strain gage elements electrically connected to form a Wheatstone bridge circuit
This simple circuit can detect the invisible fields of voltage which surround all electrified objects. It acts as an electronic “electroscope.”
Guarded resistance bridge
By using a guarded scanner and two sources to form a guarded resistance bridge, measurements from 100KW to 10GW can be made with excellent accuracy. This system provides a simple yet effective way to activate both the high and low guard circuits.
The guard ring functions as follows. Because the DAC output voltage is similar to the op amp’s inverting input voltage there is essentially no potential difference between the guard ring and summing junction and its associated components inside the ring. The ring intercepts outside surface leakage currents where they are shunted by the low impedance of the DAC.