Blocking Negative voltages to ADC

Hi..
One of the signal which is ( -3.3V to 0 ) need to clamp to level of 0V.
I can do that using the level shifter by using a voltage divider pulled up to Vref; but need to implement some other technique.

Basically I want to block (-3.3V to 0 ) range to ADC and allowing only ( 0 to +3.3VDC) signal to process.
ADC is referenced to +3.3VDC and negative reference is tied to GND internally.

Any idea or advise to work in this area?

Regards.

dpj said:

Hi..
One of the signal which is ( -3.3V to 0 ) need to clamp to level of 0V.
I can do that using the level shifter by using a voltage divider pulled up to Vref; but need to implement some other technique.

Click to expand...

hai.......... you are bit confusing.. you want to clamp negative voltages to 0V or shift -3V to +3V? If it is clamp, how can you achieve with level shifter by using voltage divider?

Regards,
karthikkr

I don't get your problem clearly.

If the input is -3.3V to 0V and you want the ADC to read that as 0V to +3.3V, you can use a differential amplifier [with gain = 1 (unity gain)]that has one input connected to ground and the other to the input. This will give you the difference between ground (0V) and the input source which will always be positive.

Another situation is: the input is something like -3.3V to +3.3V and you want the ADC to read only the positive portion (when the input is positive). You can use a diode connection between the ADC input and ground (cathode to ADC input, anode to ground) to clamp the negative portion to -0.6V (or -0.7V), ie -Vf of the diode. If you use a schottky, it's clamped to -0.3V.

Another way is to use the PIC's comparator. The -ve input of the comparator goes to ground. The +ve input goes to the signal input. When the output of the comparator is +ve, signal input is +ve, when the output of the comparator is -ve, signal input is -ve. Have a differential amplifier at the input as stated above, so that the voltage is always +ve. When output of the comparator is +ve, take ADC reading (since input is +ve). When output of comparator is -ve, don't take reading as input voltage is -ve.

Another situation is: the input varies from -3.3V to +3.3V and you want to take the magnitude of the voltage. Then all you need is a differential amplifier as explained in the first scenario, since all voltages now will only be differences from 0 (due to the differential amplifier) and thus the magnitude of the input voltage.

Hope this helps.
Tahmid.

---------- Post added at 13:47 ---------- Previous post was at 13:45 ----------

You can find quite a few clamping circuits here:
Clamper (electronics) - Wikipedia, the free encyclopedia

Hey Tehmid,
Thanks for the reply.
I am sorry for the little vague description.

Actually my signal span is -3.3V to 0 +3.3V Dc which is expected to follow PWM of 20kHz.
I want to block negative part of the same and intend to use Schottky Diodes ( A ADC Pin and Ground ) for the same as at very low current drop will be negligible.
Thanks for your other suggestion.

Regards.

hiiiii......
i am electrical engineering student, and in my last semester, i have a problem with ADC in pic16f877a.. .my project is a digital weight scale using load cell sensor ..i have amliplfied the output voltage from this sensor using AD620(Instrumentation amplifier) and i want to read the output by using ADC in pic16f877a...i used hyper-terminal prog. but i have a big problem with negative output voltage ..a fast drift in output voltage of the sensor occur and this causes decreasing in voltage till reaching negative ..how can i invert the negative output to positive...and if u have a code for ADC using pic16f877a..
i will be grateful if u help me.
& thxxxx