Blocking Negative voltages to ADC

[dpj]

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.

 

[Boncuk]

Use a Schottky diode to block the negative voltage.

At low current the diode's forward voltage drop should be negligable. Otherwise correct the value in the software.

Boncuk

 

[dpj]

Thanks Boncuk..

 

[crutschow]

It would be better to connect a series resistor to the input (say 10k) with a Schottky diode (cathode) to ground. That will avoid the offset voltage of the diode. The diode will limit the negative voltage to about 0.4-0.5V which should cause no problem.

What is the highest frequency of the voltage you are converting?

 

[Nigel Goodwin]

It would be better to connect a series resistor to the input (say 10k) with a Schottky diode (cathode) to ground. That will avoid the offset voltage of the diode. The diode will limit the negative voltage to about 0.4-0.5V which should cause no problem.

PIC's already have suitable diodes internally, so no need for an external one.

An obvious conversion method would be use an opamp to invert the signal to make it 0 to +3.3V rather than negative.

 

[dpj]

The maximum frequency can go upto 20KHz..
Thanks crutschow..

 

[dpj]

Sorry guys..
I wrongly mention -3.3V to 0 as signal swing.
Actual swing is -3.3V to 0 to +3.3V.
And I want to block that negative part of it.
Thanks for help buddy!!

 

[ericgibbs]

Sorry guys..
I wrongly mention -3.3V to 0 as signal swing.
Actual swing is -3.3V to 0 to +3.3V.
And I want to block that negative part of it.
Thanks for help buddy!!

A simple 1K resistor in series with the ADC input is all thats required, the internal diodes will do the rest.
During the small [-0.7v] negative swing due to the internal clamp diode turn on voltage, the ADC will reads zero counts.

 

[dpj]

Hi Ericgibbs!!

I am using 56F8037 Freescale Micro controller and data sheet recommends ADC Input as -0.3 to 4V as Analog Input Voltage Range.
I cant see details about internal diodes in 56F8037

Regards.

Datasheet link
https://www.electro-tech-online.com/custompdfs/2012/01/MC56F8037.pdf

 

[ericgibbs]

hi,
Clips from the datasheet.

 

[dpj]

Hey Eric,
Thats really useful.
Will these diodes work precisely at 20 KHz changing signal level??
This signal is being fed by a 20 KHz PWM.
Regards.

 

[ericgibbs]

Hey Eric,
Thats really useful.
Will these diodes work precisely at 20 KHz changing signal level??
This signal is being fed by a 20 KHz PWM.
Regards.

hi,
I see that for your MCU that the continuous clamp current is 2mA, so for a -3.3V swing below zero I would use a 2k2 series resistor.

What is the maximum sampling rate for your MCU.??

 

[dpj]

Hi..
I intend to work with sampling rate of 20 KHz..
Its related to some other functionality of the device.
Regards.