Op Amp Power Rails

[richard.c]

Hi,

Wondered if someone could give me a theoretical answer to a little problem, as I am having difficulty in getting accurate voltage test readings to prove the point.

I'm using a couple of op amps LF356 and TL084 to amplify the signal up to +1.7 0 -1.7 and then offsetting it to produce 0 to +3.4v. (sensor input - very slow change)

The testing power supply is nominally - + 5v , but would like to reduce this to a +5v only in the final version by using an ICL7660 converter - however with the load of 12 ma the output of this chip drops to 4.43volts - confirmed by its datasheet.

What I cannot accurately measure when using the 7660, is if there is any difference on the signal being amplified when running at this lower voltage - is the reduced negative voltage supply actually reducing the gain of the signal ?
Also would it actually affect the lineararity of the signal ?

Similarly if I was to pop a diode in the +5v rail to drop it to 4.4 volts would the gain be proportionally reduced ?

Will have to do that electronics theory course one day !


Thanks

Richard

 

[crutschow]

In general, although the op amp open loop gain may change with voltage it's typically so high compared to the closed loop gain that the closed loop gain won't vary appreciably.

I would need to see the circuit to answer more definitively.

 

[audioguru]

The minimum recommewnded supply voltage for the LF356 is 10V. Its inputs are not guaranteed to work if they are within 5V from the negative supply voltage.

The minimum recommended supply voltage for the TL08x is 7V. The inputs are not guaranteed to work if they are within 4V from the negative supply voltage.

The lousy old LM358 dual and LM324 quad opamps have a minimum recommended supply voltage of 3V. Their inputs are guaranteed to work at the negative supply voltage. They are low current, low bandwidth, noisy and have crossover distortion.

Maybe you can use the much better MC34071 single, MC34072 dual or MC34074 quad opamps that also work from a supply as low as 3V.

 

[Speakerguy]

I recommend the Texas Instrument's TLV2462 single supply rail-to-rail in and rail to rail out op amp as a general nice to have around little unit.

https://focus.ti.com/docs/prod/folders/print/tlv2462.html

You don't need all of its capabilities, but it will more than suffice for your +5V supply application. Also notable is its ability to drive a ridiculous 80mA. It comes in a wide variety of packages (dual, quads, singles, SOIC, DIP, MSOP, TSSOP, SOT-23).

TI will send you some for free, or you can buy them for about 2 bucks apiece at Digikey.

 

[Speakerguy]

Nevermind, I misunderstood your question.

 

[richard.c]

Hi Guys,

Thanks for all that, it is a circuit I have modified rather than designed - but I think the keynote is that I must run it at -/+ 5v or risk errors by allowing it to go below that.

cheers

Richard