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Precision Amplifier with Digital uC Control

When Instruments are designed a analog front end is essential and also as most equipment have digital or microcontroller interface the analog circuit needs to have digital access. The Circuits DACT0008 and DACT0009 are both useful in building instruments which have digital control.
This circuit DACT0009 is similar to DACT0008 but gains of upto 100 can be realized in this configuration, this is useful for signal conditioning of low mV outputs of transducers. The gain selection resistors R3 to R6 can be selected by the user and can be anywhere from 1K to 1M and can also be trimpots for obtaining gains as required by user, the resistor values shown are for decade gains e.g. for an auto ranging DPM.

Precision Amplifier with Digital Control

R1 and C1 reduce ripple in input and also snubs transients, ZD1 and ZD2 Zeners clamp input to +/- 4.7V the input current is limited by R1 lastly C1 and C2 are decoupling capacitors. The OpAmp U3 is used to increase the input impedance so that very low mV inputs are not loaded on measurement, the user can terminate the inputs with a resistor of his choice like 10M or 1M to avoid floating of the inputs when no measurement is being made. U5 is used as an Inverting buffer to restore polarity of the input and U4 is used as a buffer on the output of 4052 because loading it by resistance of value less than 1M will cause an error. An alternative is use R7 = R8 =1M and remove U4 but this may not be ideal. Gains of greeter than 100 may not be practical because at 100 gain itself a 100uV offset will be around 10mV at the output (100uV*100) this can be trimmed using the offset null option in the OP07, connect a trimpot between 1 and 8 and connect wiper to +5.

Precision Amplifier with Digital Control

For better performance use ICL7650 ( not pin compatible ) instead of OP07 and use +/- 7.5V instead of +/-5V supply.

Eight steps for gain or attenuation can be added by using two 4051 and by using Pin 6 Inhibit on 4051/52 limitless steps can be added by cascading many 4051,52,53 as Pin 6 works like a chip select.

Some extended applications of this circuits are……. Error correction in Transducer amplifiers by correcting gain. Auto ranging in DMM. Sensor selection or Input type selection in Process control. Digitally Preset power supplies or electronic loads. Programmable Precision mV or mA sources. PC or uC or uP based instruments. Data loggers and Scanners.

Voltage to Current Convertor using LM723

his Circuit converts a voltage control output from a Process Controller to be converted into a Current Control if the AC-Drive or Valve needs a Current Control Signal.

Significance of Current Loop 4 to 20 mA Standard

Voltage to Current Convertor using LM723

This is a three wire voltage to current loop converter. The 1-5 V DC is attenuated and fed to pin 5 LM723 opamp section which tries to maintain the same voltage at pin 10 across the 10 E, thereby producing a open collector constant current sink proportional to the 1-5V input. By trimming the attenuator you can scale-calibrate 1-5V input to 4-20mA output for looping many instruments in series, like a controller, recorder or PLC. With a supply voltage upto 24V, three instruments can be looped. The connection to pin 6 is required to convert 0-1 input to 4-20mA.

All the transmitter circuits can be seen here. Industrial Process Control Circuits

This circuit was designed by me in the eighties, the 555 was for negative supply, The whole thing went into the anodized cast aluminuim head of a sensor.

How 4-20mA Works

Using Thermocouple with DMM or DVM

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’.

Digital gain control of Opamp.

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.

Digital gain control of Opamp

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.