# Voltage amp solution needed

Discussion in 'Electronic Basics' started by Pete, Aug 9, 2003.

1. ### PeteGuest

Hi Group

Ive recently purchased a 10-bit resolution data acquisition card with a
range of -1V to 1V sampling at upto 20MHz.
Upon running a few tests ive realised that the raw voltage signal that i am
measuring varies between around -20mV and 20mV. My problem is that the
resolution in that range is not all that great and i fear it is too late to
exchange for a 16-bit card (which would do the job at a much higher cost).
Is there a basic solution where i could construct or purchase an amplifier
off the shelf that would condition the raw signal to use the entire span of
the card(-1V to 1V) and is also capable of performing at frequencies upto
20MHz?

Thanks

Peter

2. ### John PopelishGuest

You can build a fairly simple 5X ampifier that is pretty flat up to 20
MHz with something like an LM7171 (\$2.80 from digikey) if you can come
up with a positive and negative supply. The offset specs are not
perfect, but you can measure that with your A/D converter board and
subtract it from the other measured results.

3. ### John PopelishGuest

The video line driver on page 16 of the data sheet:
http://cache.national.com/ds/LM/LM7171.pdf

gives the general idea, except that the two resistors in the voltage
divider to the - input have to have a ratio of 4 to 1, instead of
being equal, to give an overall gain of 5 (say 510 to ground and 2k
feedback, with, perhaps a 100 ohm trim pot between them (with the
wiper to the - input) as a gain trim. And I am assuming that you
would put this right next ot your A/D input, so no series terminating
resistor would be needed on the output.

Read the application notes section for how to bypass the supply lines
for such a high frequency opamp.