# The IN0 pin of ADC0808 shows 2.5volts with no connection to the sensor

#### KumaraGuruparan

Joined Aug 20, 2007
19
can anybody tell me what value should i put for R3, R4, R5, R6, C3, C4?

#### nanovate

Joined May 7, 2007
666
What opamp are you using? Does its input/output include the ground rail? The LM324 is one that does that. If it doesn't include the ground rail or you are not using split supplies then the minimum output voltage will not reach zero. The values you mentioned should be ok although you could set R5 and R6 for a gain of 5 to 7.5. You never mentioned the temperature range of interest.

#### KumaraGuruparan

Joined Aug 20, 2007
19
What opamp are you using? Does its input/output include the ground rail? The LM324 is one that does that. If it doesn't include the ground rail or you are not using split supplies then the minimum output voltage will not reach zero. The values you mentioned should be ok although you could set R5 and R6 for a gain of 5 to 7.5. You never mentioned the temperature range of interest.
I m using MCP604 op amp. The temperature range is 0-250deg centigrade. I have tested the circuit u have mentioned. i m getting voltage range 0.73-1.04 volts. i need voltage range 0 to 2.5 volts....

#### nanovate

Joined May 7, 2007
666
What is the voltage coming out of A3? For your temps it should be from 100mV to around 200mV. Once you have established that your current excitation circuit works and you are getting the proper voltage out of A3 then you can add some gain (and offset) to get your desired range.

#### KumaraGuruparan

Joined Aug 20, 2007
19
What is the voltage coming out of A3? For your temps it should be from 100mV to around 200mV. Once you have established that your current excitation circuit works and you are getting the proper voltage out of A3 then you can add some gain (and offset) to get your desired range.
When the RTD is at 0 deg the o/p of A3 is 0.70 v and at 250 deg it is 1.04V. I m trying to change this range as 0-2.5 V... Do u have any idea??

#### nanovate

Joined May 7, 2007
666
Have you tried to troubleshoot your circuit?
When the RTD is at 0 deg the o/p of A3 is 0.70 v and at 250 deg it is 1.04V. I m trying to change this range as 0-2.5 V... Do u have any idea??
If your RTD is supposed to be 100 Ohms at 0C then you should get 100mV at 0C. Are you sure you hooked up everything correctly?

#### KumaraGuruparan

Joined Aug 20, 2007
19
Have you tried to troubleshoot your circuit?
If your RTD is supposed to be 100 Ohms at 0C then you should get 100mV at 0C. Are you sure you hooked up everything correctly?
yes i have connected everything correctly. the problem is i m using a 8 bit controller. The change in resistance in the rtd is very small. I think this small change does not provide large voltage variation(i need 0-2.5v but i get 0.78 to 1.04v). this voltage does not change the counts to a large extent and i m not getting the desired output

#### nanovate

Joined May 7, 2007
666
The change in resistance in the rtd is very small. I think this small change does not provide large voltage variation(i need 0-2.5v but i get 0.78 to 1.04v). this voltage does not change the counts to a large extent and i m not getting the desired output
That is why earlier I said:
How much resolution do you need? 8-bits may not be enough.
Usually I see a minimum of 12bits used. RTDs are expensive and unless you are only looking at a small band of temps the 8-bits are not enough.

Once you have established that the basic circuit is working then you can gain and offset the signal to match the input range of your ADC. I think that is strange that you are not gettng around 100mV across the RTD at 0C. If you put a 100 ohm resistor instead what is the output of A3?

#### KumaraGuruparan

Joined Aug 20, 2007
19
Once you have established that the basic circuit is working then you can gain and offset the signal to match the input range of your ADC. I think that is strange that you are not gettng around 100mV across the RTD at 0C. If you put a 100 ohm resistor instead what is the output of A3?
I m getting 0.16V at 0 degree temperature. The 0.78V i said is at room temperature. can u please send me or explain me the gain circuit.

#### iraqi

Joined Dec 9, 2010
1
i have same problem, but i am sure my software is correct. but i get random result, so i think you must use power greater than 5v and drop it down to 5v by regulator.
this make the IC more stable.
i think that.