ADC and small output pressure sensor - Page 1
Forum section: electronics / beginners

author: Griffin
author_rank: Unknown
timestamp: May 24, 2012, 08:16:58 pm
content: Hi AllI am working on another side project again this time involving pressure sensors. I have a non pre-amplified pressure sensor from Honeywell;http://za.rs-online.com/web/p/products/2355891As always I have a problem. The pressure sensor has an output of 145mV to 250mV with a sensitivity of 0.85mv per psi. Now from my limited knowledge that is too small to measure with the ADC in 10bit mode directly as far as I understand so what can I use to amplify it so that the ADC can read those values from 1-5v or another solution that will give me that accuracy close to 0.85 per increment.PS I have seen a operation amplifier used but that would require a dual rail power supply and I am hoping to avoid that for just this one sensor. I have all 8 ADC pins available on an ATMega324A.

author: ejeffrey
author_rank: Super Contributor
timestamp: May 25, 2012, 03:07:31 am
content: There is no requirement for an opamp to have +/- rails. They only have +ve and -ve. The input and output signals must be within a specified range relative to +ve and -ve, but either one can be ground. Some amplifier circuits require split supplies, but for your application that is unnecessary. A simple 10x non-inverting amplifier will rescale your signal from 1.45 V .. 2.5 V. That is probably good enough, but if you need more resolution you can shift the level down and add more gain.

author: w2aew
author_rank: Super Contributor
timestamp: May 25, 2012, 07:21:15 pm
content: Agreed with the opamp idea. There are PLENTY of single-supply rated opamps that include rail-rail input and outputs, that can easily be used to offset and amplify this sensor signal to create an output signal that will more effectively drive the full input range of the DAC.

author: MikeK
author_rank: Super Contributor
timestamp: May 25, 2012, 07:50:41 pm
content: Is there a name given to this spec of how close it can get to the rails? I've looked through, say, Mouser before and not been able to find an op-amp that suited my application.

author: sonicj
author_rank: Frequent Contributor
timestamp: May 25, 2012, 08:04:13 pm
content: Quote from: MikeK on May 25, 2012, 07:50:41 pmRail to Rail

author: MikeK
author_rank: Super Contributor
timestamp: May 25, 2012, 08:07:18 pm
content: Quote from: sonicj on May 25, 2012, 08:04:13 pmI meant a technical term, like what to look for in the datasheet.

author: T4P
author_rank: Super Contributor
timestamp: May 26, 2012, 03:37:12 am
content: Quote from: MikeK on May 25, 2012, 08:07:18 pmThey always say "Rail to rail" on the topI have seen RRIO opamps but most of them would be RROPI just saw a 190MHz RRIO opamp on E14 for 6$ ...

author: Griffin
author_rank: Unknown
timestamp: May 26, 2012, 02:17:18 pm
content: From what I understand in the AVR Freaks forum they mentioned connecting the sensor directly to 2 ADC pins on the ATMega324A and using what's called differential mode. This could then be used with a x10 gain to give me a similar result. Will this work or will the op-amp be the better bet?

author: Rufus
author_rank: Super Contributor
timestamp: May 26, 2012, 03:31:36 pm
content: Quote from: Griffin on May 26, 2012, 02:17:18 pmWhat works depends on how accurate you require it to be, if you will do software calibration, and your available supply voltages. The sensors you linked are a bit cheap so if you want accuracy over temperature you need some other external circuitry as well.

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 26, 2012, 08:09:53 pm
content: Quote from: Griffin on May 26, 2012, 02:17:18 pmI'm not an AVR specialist but looking at the datasheet you can set the internal reference to 2.56 volts and double your resolution.Not a problem since the maximum you will measure is (.25-.145)*10 = 1.05 volts.Too bad the other internal reference of 1.1 volts "should not be used" in differential mode, although they don't say it can't be used.Would be nice to test, gives you full resolution