RE: Thoughts on Scaling and Biasing an ADC Input Signal

shabaz — Sun, 17 May 2020 19:32:36 GMT

Hi Andrew,

I was going to say usually ADS1115 is an easy justification because it is low-cost, but it doesn't seem to be that any more, at least for one-offs. I don't understand TI pricing.. a circa £20 dev-board for a different chip I was looking at is now over £200..

There's no need for negative supplies or negative voltages to be buffered (or were you discussing the difference between two positive voltages?), the differential inputs can be driven from dual op-amps, known as a differential driver (example is ADA4941 but there are probably lower-cost alternatives, or it could be formed from two op amps). That will generate differential outputs centred around a value you choose based on an input reference voltage. For example, if your ADC only accepts 0-2.048V on its inputs, then you could use 1.024V as a reference. Then, your input, single-ended, between 0 and any positive voltage, will (theoretically) use the full range, based on the gain value you set (caveat; the op-amps won't completely go rail to rail, so you'll lose some range). By differential output I mean each of the two signals will be positive (i.e. between 0-2.048V for instance) but subtracting one from the other gets you the -2.048 to +2.048 range, and no negative supplies are used.

However, if you just want to measure between 0V and 5V, it could be easier to just use a single ended ADC and if you definitely need the extra resolution you could choose (say) an 18-bit ADC, if all else is equal (which it rarely is!). I'm not sure I ever end up picking the 'right' ADC. But for 'just a redesign' for the input circuitry or references, something better or cheaper always seems to be missed : (