Precision Attenuator for 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.

Precision Attenuator with Digital Control

The Circuit DACT0008 is a programmable attenuator and the digital control can be a remote dip switch, a CMOS Logic Output like the A-B-C-D outputs of a decade counter, or an I/O port of a uC like 80C31.

The heart of the circuit is the popular OP07 OpAmp with Ultra Low Offset in the inverting configuration, 4052 a CMOS analog multiplexer switch enables the gain change, the innovation of the circuit is that the on resistance ( around 100 ohms) of 4052 switch is bypassed so that no error is introduced by its use.

The resistors used R1 to R6 can be 0.1% 50ppm if you will use a 3 ½ DPM i.e. + /- 1999 counts ( approx. 11 bit ), but for 4 ½ DPM ( approx. 14 bit ) you may need to have trimpots2 in place of R3, R4, R5 & R6 gain selection resistors to properly calibrate to required accuracy but for testing or trials use 1% 100ppm MFR resistors but the errors will be around 1%.

Precision Attenuator with digital control

b. Output

Output connect to DPM 7107/7135 or any other A/D Convertor or OpAmp Stage. Use a buffer at output if output has to be loaded by a value less than 1Meg. Use an inverting buffer if input leads have to have polarity where gnd is -In. See DACT0009 for details.

c. 4052 CMOS Switch

The 4052/51/53 Analog Multiplexers have an on Resistance of around 100E the highlight of the circuit is that the CMOS on resistance comes in series with the opamp output source resistance, which produces no error at output.

Digital Control Options

A and B can be controlled by I/O port of uC, like 80C31 so that the uC can Control gain. A and B can be given to Counters like 4029/4518 to scroll gain digitally. A and B can be connected to DIP switch or thumbwheel switch.

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.

3-1/2 Digit ICL7107 DPM Digital Panel Meter

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.

ICL7107 DPM 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.

Mains Voltage monitor using LM3914

This Circuit helps in the monitoring of mains supply voltage. It does not use a isolation step down transformer. This has to be constructed only by skilled people with knowledge of safety requirements.

C1 limits the current and drops most of the voltage. The zener regulated supply is for the chip. C2 can be raised to 220uF or more if required. The bar mode display may consume more power.

Mains Voltage Power Transformers 

R2-R3-R5-R6 form a voltage divider to get a sample of the input voltage, D11-C3 get the DC value.

Adjust R5 preset with a log Plastic tweaker to get the 5th led to just turn on when input voltage is at 138V AC. This has to be done after PCB is put in a sealed fire-retardant-plastic or epoxy box. drill a hole in box for plastic tweaker.

National LM3914 – Dot Bar Display Driver
This Circuit is Not a Tested Design. It is an Idea for study

Mains Voltage monitor using LM3914

Warning : This Circuit is Mains Operated without Isolation Transformer and will give lethal electric shock if touched when the circuit is turned on. Test circuit only with DC 9V Bench Power Supply to try it out. Do not use 230V AC.

Microohm Meter with LED Analog Bar

This indicates like LM3914 in dot-mode. It is a drawing i made made to troubleshoot a gadget, around two decades ago. Strangely it had a echo of a design i had made into a 7107 dpm years before that. Now i am scanning all my drawing and notes, useful or not. Clean or with errors. Many Errors = 1 Blunder. Some projects i made have been expensive Blunders. So see them with a skeptic eye, fix them, try them. Thats all for now.

See the Circuit Full Size – Microohm Meter with LED Analog Bar

Microohm Meter with LED Analog Bar

This has a 9V battery power. The 555 spins and a negative voltage for Opamp is created. This is a Low Offset amp of OP37 of Precision Monolithics, Inc PMI an early innovator. This diff-amp amplifies the uV of a 4 wire resistance measurement.

Now the current pump is the 2N2222 you see above the OP37. The FET and 555 do synchronous rectification. The LM324 is the Indicator and Analog to LED Dot-Bar Converter. The probes are Gold Plated, or use solid gold pins if you have them in plenty.

Interface ICL7135 Voltmeter to PC

A PC based Analog Voltmeter. The Printer Port or Parallel Port Interface is used to acquire high resolution analog data using ICL7135. This is a method of getting analog data into your PC. This is only of Educational Value. Applications include Data Loggers and PC based Process Monitors, chart recorders. (USB, Wireless Interfaces and Tablet Computers are in vogue today

ICL7135 to Printer port Interface

The four Digit Drive outputs and four BCD outputs of the Multiplexed Display Driver of 7135, along with A-D status output which is also four, are routed to four input pins on printer port via three 74HCT373 Digital Switches. This is because we have less Input pins on Port and 4 are shared by making one 74HCT373 transparent and others High Impedance when PC takes a reading. So the three sets of four outputs of 7135 can be read sequentially.

ICL7135 to Printer port Interface

74HCT139 U9A which is controlled by the PC program can select U3, U4 or U5 for sending data to 4 pins on printer port. Data sent on the eight output pins of printer port can be latched by U10B into U6. This eight outputs of U6 can drive relays or lamps.

The ICL7135 details can be seen here 41/2 A/D with BCD ICL7135 . This Mixed device was far ahead of its times, a product of Intersil.

Three Opamp Differential Instrumentation

This is the best Instrumentation OpAmp, Great CMRR, ensure supply has no ripple and keep analog and digital grounds separate. Ri can be replaced with a trimpot and resistor to alter gain. Connect a preset ends to pins 1 and 8 and preset wiper to VCC for Offset Null when high gains are configured.

Thermocouple and Pt-100 RTD

The Input zeners and diodes form a protective clamp for all voltages above VCC-VDD. If supply is changed to +12 -12 change zeners to 12V zeners. Use similar Zeners at output to protect Output from being zapped by overvoltages or high energy – voltage*frequency transients. Add plastic capacitors across Rf for damping AC operation or ripple. Also avoid floating inputs by providing a bias.

3 Op-Amp Differential Instrumentation Amp

Vout = (Vp – Vn) * (2Rf+Ri)/Ri

Related Reading