Need help on signal conditioning circuit

Discussion in 'The Projects Forum' started by dhanraj_kmr, Dec 6, 2008.

Dec 6, 2008
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Hi to all,
I am new to this forum. I am bulding a temp sensor that is LM35DZ. From the datasheet it seems sensor delivers 10mv per 1°c. so i cant connect this output to an ADC0808 which cant detect this voltage a s a variation. so i need to build a singnal contioning circuit? my ADC0808 Ref voltage is 3.3v from my MCU. On that ADC another node i am connecting humidity sensor.

I am confused to select sensor either thermister or LM35. which is best and easy to use? I need help.

2. SgtWookie Expert

Jul 17, 2007
22,182
1,728
An LM35 is linear across it's entire temperature range.

Why can't your ADC0808 detect 10mV change? You could set your reference voltage to 2.56v using a TL431 or the like, which would give you 10mV/bit, or 1°C per bit, no scaling needed. If your humidity sensor had a different resolution, it could be scaled using a resistive divider or amplified using a precision opamp.

A thermistor is not linear. Better to use the LM35.

Dec 6, 2008
17
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So whats the need of setting my ADC0808 ref volt as exactly 2.56 using TL431 insteed of 3.3 ref volt from my MCU?

pls explain.

4. SgtWookie Expert

Jul 17, 2007
22,182
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An 8-bit ADC has 256 discrete steps.
10mV x 256 = 2.56 volts.
Divide 3.3 by 256 and see what you'd get.

Dec 6, 2008
17
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Ya its 12mv. Even i am designing humidity ciruit using cybersen sensor. From this sensor i am getting frequency output. i need to convert it into voltage. So i've bought frequency to voltage converter IC LM2917N 8 pin ic. But the output variation seems not in voltage. Its still on the freq. whats the matter?

is my freq to volt convertor IC gone?

6. SgtWookie Expert

Jul 17, 2007
22,182
1,728
I can't tell you that offhand. You may have a grounding problem or bad/omitted bypass capacitors across the Vcc/ground connections.

If you're breadboarding it, check for bad connections. Mixing analog with digital can create huge problems, particularly if you intermix their grounds/Vcc/Vee/Vdd/Vss everywhere.

As I've said several times on this forum, combining digital and analog on a board is somewhat akin to inviting a punk band to play in a public library. The outcome won't be good without careful planning.

Meanwhile, you've provided neither schematic nor layout of what you have so far. So, right now, I'm blindfolded and swinging at a pinata.

Dec 6, 2008
17
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ya here i hav uploaded my freq to volt converter IC's Diagram.

As freq input i've given it from my sensor. My sensor has connected as Rb resis. from that i am generating frequency. Both my c and cc are 1?f and 0.01?f. Ra is 170?. Vdd is 5v. output clock is connected as freq input to LM2917N. here i've done all thing and checked my GND, VCC and all.

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8. SgtWookie Expert

Jul 17, 2007
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You attached two completely unrelated items with no values.

I have absolutely no clue what you're trying to do.

Dec 6, 2008
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pls dont wrong with me. My actual need is generating digital output using ADC0808. I need to connect two sensors to ADC. one is temp sensor Thermistr. Another one is Humidity sensor cybersen. For thermistr there are two pins which one is connected with 3.3v through 100ohm. Another pin of Thermistr is directly connected to ADC. Am i doing correctly? My temp sensor workin correctly. there is no complex circuit. Thats it from me. I dont have much power electronics knowlege thats why this kind of struggling.
Pls adjust my mistakes. Pls check my attached Word doc of my LM2917N from my previous post.

10. thingmaker3 Retired Moderator

May 16, 2005
5,072
6
Use 2.55V instead of 3.3V as SgtWookie has already suggested, and for the reasons he already listed.

What is the expected output from your cybersen device? Can you post a link to a datasheet?

Dec 6, 2008
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12. thingmaker3 Retired Moderator

May 16, 2005
5,072
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The details I can get from your attached doc say nothing of your sensor. You have asked if the VFC in your doc will work with your application, but no one can know the answer to this without details of your sensor, can they?

I wish you good fortune.

Dec 6, 2008
17
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