ADC -- How to measure negative voltage

[funghome]

I have an ADC LTC2410. How can it measure negative voltage with respect to ground, but not IN-?
IN- will be grounded in measuring.
Thanks.

[laktronics]

Hi,
Either you need to invert the negative signal and apply, or, you can level shift the signal around half Vcc, apply it to IN+ and connect IN- to half Vcc. Vref in the second case should be Vcc and the shifted signal range to be adjusted to be within 0 to 5V.
Normally, if the signal is floating, you should ground the -ve lead and connect the positive lead directly to IN+ without any problem.

Regards,
Laktronics

[funghome]

Hi,
Either you need to invert the negative signal and apply, or, you can level shift the signal around half Vcc, apply it to IN+ and connect IN- to half Vcc. Vref in the second case should be Vcc and the shifted signal range to be adjusted to be within 0 to 5V.
Normally, if the signal is floating, you should ground the -ve lead and connect the positive lead directly to IN+ without any problem.

Regards,
Laktronics

Invert the negative signal need two resistor and opamp. I think it will lost some precision.
Can you draw a circuit digram to explain more how to shift the input signal half Vcc. Input signal is bipolar voltage and common ground with ADC.
Thanks.

[laktronics]

Hi,
As you have rightly pointed out, any tampering with input is likely to affect precision and introduce additional noise. In the first place it appears the chip is not designed to handle negative inputs.
If your input voltage range is +/-5V, you may use two equal metal film resistors of say, 10kvalue, connected in series, with Vref(5V) connected to one end and Vin connected to the other end. Connect the mid point of the resistors to IN+. To IN- connect the mid point of another pair of series connected equal resistors with their free ends connected to Vref and ground. You have to ensure that all the four resistors have matching characteristics.

Regards,
Laktronics