In the circuit, use only metal film resistors (MFR) of 1 per cent tolerance, as this is an instrumentation application. Power supply should be a stable +5V, -5V supply, for which one can use 7805 and 7905 regulators.
The inputs TC+ and TC- terminals should go to a 4-way barrier terminal block, the 2 extra terminals are used to mount TH1 Cu thermistor. This forms an isothermal block, which is good enough.
A simple way to make a TH1 Cu thermistor, is to take a 1 Meg-ohm 2W resistor as a former and wind 2 meters of 46 SWG enameled copper (Cu) wire (5.91 ohm/meter) over it. This gives a 12-ohm value. Terminate wire ends on resistor leads.
Thermocouple Temperature using DPM or DMM
Test and Calibration –
For calibration, you will need a DMM-DPM and a milli-volt source (as shown in the Fig.). First connect source to terminals TC+ and TC-, then set source to 0.00 mV (verify with DMM for zero). The output across +out and -out (use DMM) terminals must be mV representing the room temperature (RT). For example, if RT is 30° C (use a glass thermometer) then +out should be 30mV at 0mV input. Adjust VR1 till 30mV is read at +out terminal. This is ‘zero cal’.
This is a Digital Panel Meter based on ICL7107. This was the workhorse Digital Readout chip before Low power uC designs were developed. Even now it is the easiest way an analog parameter can be displayed in an instrument.
Instrumentation and Measurement Circuits
3-1/2 Display is just enough resolution for recording process data or troubleshooting equipment. With 4-1/2 you notice the problems of thermoelectric EMF of probes and connectors. That is one reason gold plating is used another being low contact resistance. In 4 1/2 the reading can change due to contact resistance too, if the input impedance of the analog signal conditioning is low or the protection diodes, RC filter caps are leaky.
When you measure an analog parameter from a sensor; you attenuate or amplify it, to scale to the A/D converter range. You also need to protect or isolate the analog front end from high energy mis-connections, which will happen during field use. The operator should not get injured from any high energy leakage while measuring.
Resolution is the finest detail you can enumerate in a parameter or object. You can say the crowd was 3000 or 4000 people. Here 1000 is the resolution, finer data does not matter here. You would also hear things like, there were 80 to 90 boys in that classroom. 10 is the resolution there.
Accuracy is how precise a statement or number is. If there was 85 boys in that class, the above statement is accurate. If there was 65 boys, the measuring method or process needs calibration or examination.
In this circuit, the plastic caps should be of very low leakage. Multilayer plastic caps are good for instrumentation.
The gain of U1 can be controlled by a digital binary 1248 nibble at ABC. The gain at digital 000 is unity or 1 and the gain at various stages are set by 4051.
Precision Attenuator with Digital Control – delabs
There are eight different gains as the steps of gain resistor network is chosen by 4051. The on resistance of 4051 channel around 100E gets added to U1 pin 2 internal impedance.
Auto ranging 4-1/2 Digit Digital Voltmeter – delabs
You can use separate resistor networks with trimpots for each channel if you require but keep the networks total burden on U1 pin 6 to around 10K, not less than than. You can use this to set the gain of a amplifier with the help of a microcontroller.
A sample and hold is like an analog memory. If The digital control A is low 4066 switch is open, and when A is high switch is closed. U2B is a buffer so as to ensure quick charging of C1 thru 4066 on resistance of 100E.
Mixed and Interface Circuits
U2A is a FET input opamp buffer which does not load or drain the cap C1. When A goes high the input analog sample is stored in C1. A has to be high for say 10*1uF*100E = 1mS, so that a proper stable sample is stored. When A is low C1 undergoes very slow discharge as opamp input resistance and 4066 off resistance is in giga ohms. The accuracy of reading Vout falls with respect to time due to leakage currents.