"Differential output transducer" into "single ended ADC"? - Page 1
Forum section: electronics / beginners

author: king.oslo
author_rank: Frequent Contributor
timestamp: November 06, 2012, 05:03:59 pm
content: Hello there,I am interested in the MPXM2202A differential output pressure transducer. Its VOUT-range is aprox 1mV-41mV as a function of pressure: 20kPa to 200kPa. Here is the datasheet:http://cache.freescale.com/files/sensors/doc/data_sheet/MPX2202.pdfIs it possible to ground the -VOUT-output, and feed the +VOUT-output into one of the single ended inputs of my ADC? I have a voltage reference on my board, and could scale it down to aprox 41mV and feed it into VREF+ of the ADC. Will this work? Or am I going to have trouble connecting -VOUT-output to GND?What is your advice?Thank you for your time.Kind regards,Marius

author: Kremmen
author_rank: Super Contributor
timestamp: November 06, 2012, 05:44:08 pm
content: You might get away with it electrically. Hard to say from the data sheet because there didn't appear to be a detailed schematic.However, my recommendation would be to feed the sensor data to an instrumentation amp and use its output as the input to A/D conversion.The other reason besides the question above is that 40 mV is a bit low for full scale signal. Sure you can do it, but you take all kinds of risks with interference and noise, not to mention possible issues with the A/D converter on such low absolute voltages.A cheap rail to rail amp like the AD8226 will set you back less than 3 bucks in unit quantities and less than 1½ in 1000 lots. It has gain adjustable with a single resistor - as is typical of instumentation amps - and a gain of say 100 would bring your voltage up to an easy 4 volts. Or any other voltage that you might want up to 36 volts max. I use that chip here and there and it sure makes life easier. There are others as good and better, of course.

author: andyturk
author_rank: Frequent Contributor
timestamp: November 06, 2012, 06:25:58 pm
content: I'd look for another sensor, if I were you. Those things require a 10V supply and only put out a 40mV signal, so not only are you going to have to buffer and translate the output, but you'll also have to come up with a clean 10V power source.If you don't really need the 200kPa range, the MPXV6115s are good to 100kPa, run on 5V and have a full-scale output range. The MP3V5050s are 3V sensors with a 50kPa range. They're much easier to work with. I've got half a dozen of them on my bench at the moment.

author: king.oslo
author_rank: Frequent Contributor
timestamp: November 06, 2012, 06:33:30 pm
content: I learnt from some other Freescale documentation that the minimum voltage for mpxm2202a is 3v3. If you check the datasheet, you will find that it doesn't specify a minimum voltage.Do you have some additional information that I do not have?Thanks.M

author: andyturk
author_rank: Frequent Contributor
timestamp: November 06, 2012, 06:44:14 pm
content: Quote from: king.oslo on November 06, 2012, 06:33:30 pmThe datasheet you linked specifies 10V as the typical supply voltage and the chart in Figure 2 (output vs pressure) also references a 10V supply.

author: Kremmen
author_rank: Super Contributor
timestamp: November 06, 2012, 06:47:41 pm
content: It says "ratiometric to the supply voltage" so the supply needs to be stabilized and/or referenced to the A/D reference voltage. The supply is typically 10 volts but no strict minimum is specified. The 40 mV full scale is relative to 10 volt supply, so if it works with 3.3 V you are going to get an awful tiny signal at full scale. Do consider amplification.

author: SeanB
author_rank: Super Contributor
timestamp: November 06, 2012, 06:52:07 pm
content: Ratiometric sensor. you need to use both outputs and feed them into a differential amplifier and then use the single ended output from that to drive the ADC. The full scale is temperature compensated, but it needs a very stable reference to drive the sensor.Look to see if you can use other sensors, as this one needs a stable supply and a differential amplifier with a gain of 100 or so, with offset compensation and will need calibration before use.

author: andyturk
author_rank: Frequent Contributor
timestamp: November 06, 2012, 07:04:11 pm
content: Quote from: Kremmen on November 06, 2012, 06:47:41 pmGood point. I missed the ratiometric part. So those sensors should put out a ~13mV signal when powered by 3.3V. That's at full-scale of course (i.e., 200kPa) and presumably the average pressure to be measured will be less. Even one mV of noise is going to really mess up those readings.

author: king.oslo
author_rank: Frequent Contributor
timestamp: November 06, 2012, 07:07:18 pm
content: Radiometry is a set of techniques for measuring electromagnetic radiation in optics. Is also a way of saying that the output voltage is a function of the input voltage?How do I learn whether the function is linear?Thanks for your helpM

author: andyturk
author_rank: Frequent Contributor
timestamp: November 06, 2012, 07:30:10 pm
content: The output voltage differential is proportional to the pressure differential read by the sensor. That proportion, also called "sensitivity" or deltaV/deltaP stays the same even when the temperature or the supply voltage varies within the specified limits. It's basically linear. Take a look at figures 2 and 4 in the datasheet.IMO, noise is going to be a bigger factor than nonlinearity. Also, what range of pressures are you intending to measure? Do you really need a 200kPa sensor?

author: Kremmen
author_rank: Super Contributor
timestamp: November 06, 2012, 07:50:06 pm
content: Quote from: king.oslo on November 06, 2012, 07:07:18 pmErm... the latter is raTiometry, from the latin word ratio, meaning ... ahh... ratio. Nothing to do with radiation.

author: king.oslo
author_rank: Frequent Contributor
timestamp: November 07, 2012, 07:09:22 am
content: Thanks!M

author: Kremmen
author_rank: Super Contributor
timestamp: November 07, 2012, 07:44:43 am
content: SureAnd to answer the second question about linearity: if a signal is ratiometric to something else then the assumption is that the proportionality is linear. Otherwise there would not be a defined ratio between the 2. So in other words ratiometricity is a guarantee of linearity.

author: Bambur
author_rank: Regular Contributor
timestamp: November 08, 2012, 10:21:23 am
content: Quote from: king.oslo on November 06, 2012, 05:03:59 pmI may suggest to get a look at chapter 4 in Op Amp Applications Handbookhttp://www.analog.com/library/analogDialogue/archives/39-05/op_amp_applications_handbook.html.That handbooks gives a great deal on sensor signal conditioning, ratiometric sensor details, coverting differential output signals to single-ended for interfacing to single-ended ADC's.

author: andyturk
author_rank: Frequent Contributor
timestamp: November 08, 2012, 04:45:47 pm
content: Quote from: Bambur on November 08, 2012, 10:21:23 amLink doesn't work.

author: robrenz
author_rank: Super Contributor
timestamp: November 08, 2012, 04:53:20 pm
content: Quote from: andyturk on November 08, 2012, 04:45:47 pmRemove the period on the end of the link

author: andyturk
author_rank: Frequent Contributor
timestamp: November 08, 2012, 05:30:51 pm
content: Quote from: robrenz on November 08, 2012, 04:53:20 pmDoh!Thanks.

author: hlavac
author_rank: Frequent Contributor
timestamp: November 08, 2012, 07:40:10 pm
content: Just delete last dot:here