amplifier circuit?

[siriustech]

Hey Guys,

I need a circuit to do the following:

I have a signal which varies from 4.5 VDC to 5 VDC. I need it to transform it so it varies from 4.0 VDC to 5 VDC. Current is always below 10 mA. FYI: The signal is applied to a 12-bit A/D converter. I'm trying to increase the span of the signal to minimize mathematical rounding errors in a non-floating point processor (BS2e).

Anyone have any suggestions?

 

[Skogsgurra]

A standard opamp with a gain of two and some offset will do that. Have a look in any opamp application note. National Semiconductor, Linear Technology, Texas Instruments, Maxim all have such appnotes. Just google for them.

 

[logbook]

It has to be said that I am a bit suspicious of the requirements spec. It seems strange to be scaling the full scale input range into the region 4V to 5V.

However, that is the range you have asked for so here goes... Skogs had said a gain of two and that is correct. Let’s assume you want a non-inverting gain of two. Ok then feed the input into the positive input of the opamp. Use say a 1K feedback resistor from the output to the negative input, then put a 1K resister from the negative input to a +5V reference voltage.

When you have 5V input, the other opamp input is also at 5V, so the output sits at 5V as required. When you put in 4.5V, that is a 0.5V drop from the previous situation, the output drops by double this amount because of the gain of two; hence the output is now 4.0V as required. Easy.

 

[siriustech]

Thanks skogsgurra and logbook...

You guys got me thinking in the right direction - I had a serious case of mind block yesterday.