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Friday, July 26, 2019

Universal Signal Conditioning Circuit

Here is a practical application of the dual differential amp. This circuit is for DC or slow moving DC like Strain Gauge and transducer signals.

The RC and Zeners are for protection and clamping. Why and how has been described in other posts here. The high impedance op-amp input cannot face the real world unprotected.

Universal Signal Conditioning Circuit

OP07 is always the selection due to the low offset. When the sensor generates data in mV, you cannot afford any error introduced in this front end, right. Here OP290 is used for better performance. This dual opamp is better for building a two wire 4-20mA transmitter. But Op07 is fine for a three wire transmitter which consumes more power.

Observe the Power supply decoupling caps, this is a rule and every chip needs a power cushion. The front end zeners and caps have to be low leakage in many applications, ensure you have such parts or test them.

Opamp Instrumentation Circuits

There are two trimpots, bourns multiturn PCB trimpots. One for Offsett-Null or Zero and other for Span or Gain. You have to tweak them in a alternating sequence as they are interdependent to some extent.

Now your weak transducer with a voice in mV and uV; can meet an A/D "Eye to Eye" and let the whole world know what is the Weight, Temperature, Humidity, Pressure or Moisture Content in the Harsh Real World full of uncertainties.

Thursday, July 25, 2019

Linearizing Circuit with Opamps

The thermocouples and some other sensors have an output that is non-linear. That means the mV generated by them are not in direct proportion to the parameter measured. In signal conditioning Linearization can be done with cmos fet switches or even comparators and diodes.

Linearizing Circuit with Opamps

In Embedded Systems Linearization is done with Look-Up tables on ROM and also using Math and Formulae for each segment.

The above circuit used diodes and opamps to change the gain for each segment of the curve to make it a straight line within an acuuracy of 1% or 2 %.

LM346 has programmable gain and 4 opamps in one package. The LF442 is a dual low power operational amplifier and superior for DC application in comparison  to LM358.

The about configuration increases or decreases the gain of the block. You can use each opamp and the dip-switch to change the gain. The resistor selection is important for obtaining the right gain.

Wednesday, July 24, 2019

Auto ranging 4-1/2 Digit Digital Voltmeter

Auto Ranging for ICL7107 had to be done with window comparators. In ICL7135 Digital Outputs are available for autoranging. Here in this circuit is a close approximation of what i may have designed once.

Auto ranging 4-1/2 Digit Digital Voltmeter

The below Details from Datasheet - These pins are used in this circuit to Autorange the DPM with some peripheral CMOS Logic.

Auto ranging 4-1/2 Digit Digital Voltmeter

"OVERRANGE (Pin 27) - This pin goes positive when the input signal exceeds the range (20,000) of the converter. The output F/F is set at the end of BUSY and is reset to zero at the beginning of reference integrate in the next measurement cycle.

UNDERRANGE (Pin 28) - This pin goes positive when the reading is 9% of range or less. The output F/F is set at the end of BUSY (if the new reading is 1800 or less) and is reset at the beginning of signal integrate of the next reading."

ICL7135 4 and Half A to D converter with BCD Output

The Range is Defined by QB and QC of 4029, there are four ranges. The two bit Code from this can control the decadic range of an Attenuator or Amplifier.

The Overrange and Underrange are in a OR formation with R26-27. Clock from either will Inc/Dec 4029. The Underrange Pulses are used by D2 -C8 to form a Steady Logic signal on Up/Down Control of 4029. This works in a closed loop manner, to Change the Range to match input Signal. When there are no Pulses from Either UR or OR, then that range is held.

Friday, June 28, 2019

16 Channel Analog Multiplexer

This circuit is built around the chip CD4067, now called 74HC4067, it works like a single pole 16 throw rotary switch that can be controlled digitally. Clamping protection diodes are built in. Use resistors to enable this protection.

16 Channel Analog Multiplexer

Pan and Zoom the Circuit for detailed Study here - Sixteen Channel Multiplexer

The ON Resistance of this chip is around 100 ohms and OFF resistance may be in Giga ohms. Use  74HC4067 for CMOS periphery. 74HCT4067 for a TTL environment.

16 Analog channels are controlled by a Digital User Interface circuit made of 4093 and 4029. Up/Down and Auto/Man are four Push-buttons. A LED Display to show the Channel Number that is selected.

Mixed and Interface Circuits

The output of the Mux goes to OP07 whose high input impedance and low offset enables an accurate low impedance output that can go to the A/D Convertor, PLC Unit or DPM - Digital Panel Meter or voltmeter.

Wednesday, June 26, 2019

Synchronized Sample and Hold Analog Mux

This circuit has two analog mux 4051 operating in synchrony to store an analog value in a low leakage tantalum capacitor. This makes an Analog Memory possible and overall system accuracy of around 2%, which was more than adequate for this process.

Mixed Circuits Analog with Digital

The analog value from a temperature transmitter is available for a short duration when it is collected from a rotating system with copper brushes and contacts on the shaft.

Synchronized Sample and Hold Analog Mux

Pan and Zoom the Circuit for detailed Study here - Synchronized Analog Mux 

The CA3140 with one tera ohm input impedance ensures the accuracy of the analog memory. The scanning by 4029 counter frequently refreshes and updates the analog data.

The 4-20mA signals are converted to voltage by a 50 ohm shunt 1% MFR of 1Watt rating. The clock and reset signals are coming from 12V proximity switches. This is filtered and tamed by a simple transistor circuit. That way the 5V Logic circuit is safe from the 12V external inputs.

Since both the 4051 operate in tandem the analog value in the tantalum capacitor does not discharge through the 50 ohm shunt current receiver.  The CMOS switches offer very high OFF resistance, that makes this circuit workable if properly adapted.