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Accurate multiplication or division of voltages by voltages?

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uoficowboy

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Hi - I'm interested in a circuit that can either multiply one voltage by another (ie V1 * V2 = V3) or one that can divide one voltage by another (V1 / V2 = V3).

In the multiplication case, V1 and V2 are measured voltages in the circuit. In the division case, V1 is a measured voltage while V2 is a generated voltage (generated via a DAC). I could also use a digital pot instead of a DAC, if that would help. I do not expect it to change very often (though it'd be nice to preserve the functionality of changing it often).

I am hoping to achieve very high DC accuracy (hoping to get 12 bits out of the system). The output will be fed back into a control loop - so higher bandwidth is preferred (100KHz+ ideally) (hence why I don't want to do the conversion in a microcontroller). Low cost, ideally.

I can't seem to find anything that can meet these requirements, or that even comes very close to them.

Any suggestions?
 

crutschow said:
Have you looked at dedicated analog multiplier ICs such as those from Analog Devices?
Click to expand...
Yes - but I don't think they're intended for my application. They're all rated up into the MHz and GHz and cost $30+ (I'm hoping for an electrical BOM cost of less than that). They all have significant error, too.
 

There are MDACs (multiplying DACs), so if one of your "analog"
voltages has a DAC precursor maybe this is what you want.
 
dick_freebird said:
There are MDACs (multiplying DACs), so if one of your "analog"
voltages has a DAC precursor maybe this is what you want.
Click to expand...

I think an MDAC is just the same thing as a digital pot, right? I don't think that will get me what I need in this case, unfortunately.

---------- Post added at 00:43 ---------- Previous post was at 00:41 ----------
pstuckey said:
There's a few in this table around $5, but they have 1-2% error. The AD632 looks close to what you need (0.5%), but costs $12.

https://www.analog.com/en/special-linear-functions/analog-multipliersdividers/products/index.html
Click to expand...
Exactly - 1% error is a lot for me, unfortunately. And to get to that $12 price you have to be buying a whole lot of them...

Maybe the part I need doesn't exist? I might have to get clever here! :(
 

uoficowboy said:
I think an MDAC is just the same thing as a digital pot, right? I don't think that will get me what I need in this case, unfortunately.

.................................

Maybe the part I need doesn't exist? I might have to get clever here! :(
Click to expand...
An MDAC is more complex than a digital pot. It has an accurate output voltage based upon the digital input and and analog reference voltage. In a multiplying DAC you vary the reference voltage which thus "multiplies" that value by the digital setting. In general multiplying DACs are more accurate and have higher digital resolution than digital pots.

Yes, you may need to get clever. Why do you need such high accuracy in such a high frequency feedback loop? Can you separate the required high frequency accuracy from the DC accuracy?
 
crutschow said:
An MDAC is more complex than a digital pot. It has an accurate output voltage based upon the digital input and and analog reference voltage. In a multiplying DAC you vary the reference voltage which thus "multiplies" that value by the digital setting. In general multiplying DACs are more accurate and have higher digital resolution than digital pots.

Yes, you may need to get clever. Why do you need such high accuracy in such a high frequency feedback loop? Can you separate the required high frequency accuracy from the DC accuracy?
Click to expand...
Hi - sorry for the slow response. Life got in the way of my fun :)

Separating the DC accuracy from the high frequency response is exactly what I'm thinking I might try and do - use one of these Analog Devices chips for the high frequency, but wrap a lower frequency digital control loop around that to get the DC accuracy up. Not perfect, but life never is!

I still don't see how an MDAC is different from just varying the input voltage on a digital pot. In either case, isn't it true that Vout = VREF * [DAC code]/(2^n - 1) where DAC code is your digital input to the DAC and n is your number of bits?
 

uoficowboy said:
..............................

I still don't see how an MDAC is different from just varying the input voltage on a digital pot. In either case, isn't it true that Vout = VREF * [DAC code]/(2^n - 1) where DAC code is your digital input to the DAC and n is your number of bits?
Click to expand...
Yes they are similar but an MDAC can have more control bits and thus more digital resolution as compared to a digital pot. If you don't need high digital resolution, then a digital pot should work for you.
 

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