capacitance to voltage circuit for ADC

[garg29]

capacitance to voltage converter

Hi friends,

I'm having a sensor which is having a capacitive output. Which varies from 0.085 nF to 0.700 nF. I need to change this capacitive range into 0-5 Volts range can anyone please tell me how can i do it.

Thanks a lot,
With best regards,
Amit

 

[solom]

capacitive sensor lm555


i will tell you some idea from me that you can, you can use this change in capcitor to change pwm with 555 chip (it depend on r &c)
if the change in capcitor small you can make capcitor amp with op-amp

 

[garg29]

thanks for replying can u provide me the capacitor amplifying circuit

 

[WinterInJuly]

Read LM555 datasheet, you will see the astable or mono stable circuit. Replace the Capacitor by your sensor. The output frequency varies and depends on your sensor . It is simple but not very accurate. You should use the R, C 0.1% error

 

[luben111]

555 solution is not so precise.

Go to www.qprox.com - they sell capacitive sensors. They have some chips that just output the measured capacitance.

Because they use charge transfer technology the sensitivity is tremendous - you can detect approaching of human from 30-40 cm !!! (if you use big electrodes).

Other solution is to use Analog Device capacitance to voltage converter - it's high accuracy (and high price as well), I'm sorry, I forgot the part number.

best regards
luben

 

[WinterInJuly]

have a look at AD7745/AD7746 (Analog Device) which is use to measure capacitive sensor, high accurate

 

[avinash]

you cn use switch capacitor circuit for cap to voltage change.

 

[garg29]

Thanks everyone for replying. Hi, Avinash, what's a "switch capacitor circuit".

Thanks a lot,
With best regards,
Amit

 

[WinterInJuly]

You can refer to Swich Capacitor Technology

 

[smxx]

in a cap. i=c dv/dt or v =1/c∫i dt for i=constat I.
1)v=1/c I*t.
2)c=I*t/v
if u inject a costant current for a predefine time to a cop. you have a ramp voltage in cap pins. u can read this that proper to your cap. als you can use an diffrenator
to have proper voltge

V2=dv/dt V2=1/c I

|I
|.......
|
|_______________t

|V .
| . . slop=I/c
| . .
|___.___________t

|v2
|
|.......
|__________

T