LM6172: analog voltage devision using op amp

JIGAR PATEL2

Intellectual 615 points

Part Number: LM6172
Other Parts Discussed in Thread: MPY634, TL441

Hi All,

I have analog voltage division requirement using op amp for my application.

i.e. if V1 = 12, V2 = 2 then Vout = V1 / V2 = 6,

if V1 = 10, V2 = 2.5 then Vout = V1 / V2 = 4

if V1 = 5, V2 = 2 then Vout = V1 / V2 = 2.5

etc...

I have searched analog voltage division using op amp, but only concept is found. there is no any proven circuit i have found.

I do not want to use log / antilog circuit for the division.

can any body suggest me practical circuit?

regards,

jigar

over 3 years ago

0 kai klaas69 over 3 years ago

Guru 140200 points

Hi Jigar,

why do you want to perform this division? Can you please give more details?

Kai

0 JIGAR PATEL2 over 3 years ago in reply to kai klaas69

Intellectual 615 points

Hi Kai,

you may know about Ratiometric measurement in ADC in which result is independent of applied supply voltage & current variation. It depends only the precision & stability of reference resistance which generate reference voltage.

now, I want to do exactly same in hardware circuit.

as shown in figure, I have one temperature control circuit which is driven by pwm(250 Hz) applied to n channel MOSFET.

when MOSFET is on, heating is on & when MOSFET is off, heating is off.

I want to apply some Ratiometric method to control the temperature. please take note that we do not want to use ADC for this application.

I will measure voltage across reference resistance(VR) as well as across heating element(VH) using difference amplifier when MOSFET is on.

then I want to do (VH / VR). this result is independent of supply voltage & supply current variation.supply voltage & supply current variation will definitely occur when heating switched on due to sudden large current consumption.

next, I will amplify difference between (VH / VR) voltage & my precise threshold voltage using difference amplifier. output of this will be given to comparator which will help to turn off MOSFET.

So, I want to make this kind of fast acting temperature control circuit which is independent of supply voltage & current variation.

can you help me how to do (VH / VR) using op amps?

regards,

jigar

0 kai klaas69 over 3 years ago in reply to JIGAR PATEL2

Guru 140200 points

Hi Jigar,

VH / VR = VH / (IH x R) = RH / R

because

VH = RH x IH

(where IH is the current through the heater, R is the reference resistance and RH is the resistance of heater.)

Correct?

Kai

0 JIGAR PATEL2 over 3 years ago in reply to kai klaas69

Intellectual 615 points

Hi Kai,

yes, it is correct.

but what is the solution to sense the heater resistance(RH) using ratiometric method by using op amp?

Regards,

Jigar

0 kai klaas69 over 3 years ago in reply to JIGAR PATEL2

Guru 140200 points

Hi Jigar,

analog division can be performed by the help of MPY634.

Another option is to use a transistor array and to build two log-amplifiers and an antilog-amplifier to carry out the following calculation:

x / y = exp (log x - log y)

Kai

0 kai klaas69 over 3 years ago in reply to kai klaas69

Guru 140200 points

Also see the datasheet of TL441.

Kai

0 kai klaas69 over 3 years ago in reply to kai klaas69

Guru 140200 points

And here is another method:

Kai

0 JIGAR PATEL2 over 3 years ago in reply to kai klaas69

Intellectual 615 points

Hi Kai,

Thanks for the suggested solution.

I will check them & will try to implement.

If you found any other solution for the same then kindly update me.

regards,

jigar

0 kai klaas69 over 3 years ago in reply to JIGAR PATEL2

Guru 140200 points

Good luck :-)

Kai

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LM6172: analog voltage devision using op amp