A simple question about the gain in DAQ

[ThunderBird]

Hi, all, I posted this question in DAQ board but didn't get the answer, so I try here:

When do we need to use Gain to the signal?
For example, '5v signal with 0-10v ADC range', do we need to use gain? How about '10v(or 1v) signal, with 0-10v ADC range'?
Thanks.

[johnsold]

A simple question with a not quite simple answer.

The purpose of using gain is to "fill up" as much of the dynamic range of the A/D converter as possible without exceeding the range.

Whether to use gain in a particular situation depends on a number of factors. The peak signal value should not exceed the maximum A/D input value. How do the amplifier and the converter respond to overvoltage? Is there useful information in the instantaneous peaks or is it just noise which could be clipped without loss of data integrity? Typically I set the gain so that the peak value of the signal falls in the most significant bit or two of the A/D converter.

For your examples I would probably not add any gain to a 5 V signal on a 10 V scale. (Can the signal plus noise ever exceed 5.000 V? Is the gain exactly 2.0000?) With a 1 V signal I would consider a gain of 2, 4 or 5, but probably not 10.

Lynn

[Dennis_Knutson]

You use gain when you want more resolution of the measurement. Say you have a 12-bit DAC. That's 4096 steps so if you have a 0-10 volt range, each step is worth .00244 volts. If you change the gain so that the range is 0-5 volts, each step is .00122 volts. You may or may not need the added resolution. Also, your DAQ board has a limited number of range settings. You'll have to check your user manual to see which ranges the board supports. And, if you have a 5 volt signal, for example, changing to a 0-5 range will cause clipping of the measurment whenever the signal goes even slightly over 5 volts.

[Angelantoni]

hai jsut stumbeled upon this post,

one question.
When a DAQ card has 18 bit resolution it is applied only to the maximum rangesay (+/- 10 v)? if i am not using the full range of DAQ card(+/- 5 v) then my reolution will decrease?
I thought the 18 bit resolution is adjusted to what ever range we specify.!!

Thanx
Arun

[Dennis_Knutson]

Your VOLTAGE resolution will increase if you use less than the full range. If you have 18 bits at +/- 10 volts, you will still have 18 bits at +/- 5 volts or +/- 1 volt, etc. Because the number of bits stays the same, your voltage resoltion increases.