Divide-multiply-with-only-one-trim

  
This relatively simple, inexpensive circuit requiring one trimming operation can multiply or divide with a consistentaccuracy of greater than 1 part in 1,000. An inexpensive CMOS version of standard 555 timer chip T, in conjunction with low-drift LMll error amplifier A3, an inexpensive analog chopper switch SW, form a unique voltage-to-duty-cycle converter to produce the difficult transfer function necessary for accurate conversion.
Divide-multiply-with-only-one-trim - schematic

An unknown multiplicand voltage applied to the A3 error op amp circuit`s Y input controls the duty cycle of the timer through its pin 5 modulation input. The network between the sink-and-source output of the timer, pin 3, and the state trigger inputs, pins 2 and 6, cause the timer to oscillate. An error feedback signal from the timer`s discharge output, pin 7, represents the duty cycle. Integrating this duty-cycle signal with voltage reference REF representing full scale, and applying the result to the inverting input of A3, closes the feedback loop and insures high accuracy. Multiplier X feeds into another LMll op amp, A1, which acts as a input buffer and scaler. A third LMll, A2, filters and buffers the Z output. Between A1 and A2, the timer`s duty-cycle output modulates the analog switches of a CD4066 to achieve the desired multiplier output. To perform division instead of multiplication, reconfigure the op amp A1 circuit with the use of jumpers. Amplifier A2 isn`t required in the division configuration. To calibrate the circuit, connect the X andY inputs together and apply 10 V. Then adjust the 10-turn span potentiometer to achieve a 10-V output at Z for multiplication, or 1 V for the division configuration. Also check for zero output at a zero multiplier input. The circuit is scaled for 0 -10 V inputs and outputs with a small overrage capability, but other scalings are possible. Star grounding or a heavy ground bus should be used to reduce offset problems that are unavoidable in this design.



Recommended videos


News


    .