Can we give a bipolar signal to ADC of ATmega88?

Question

Can we send a bipolar signal to the ADC of ATmega88?

We know that an analogue signal has a positive value as well a negative value. So can an ADC take a negative value or does it just take the positive value of a bipolar signal?

Answer 1

This information can be found in the datasheet.

In the ADC section it says it can measure voltages between 0 V and V_cc. In the electrical characteristics section it says the absolute maximum for any pin except reset is -0.5 V to V_cc + 0.5 V.

Therefore, no it can't measure a negative voltage, and additionally, a negative voltage below -0.5 V is not recommended.

However you can bias the input so that it is within the 0 V to V_cc range of the ADC. For example, see the first answer to question How to read an audio signal using ATmega328?.

Answer 2

No. Just positive.

If this is a bipolar DC signal, you may be able to reduce the range and bias to midrange of analog V+ (VCCA) with an appropriate resistor divider, while taking care of any loading and supply/ground noise issues with low ESR caps on the VCCA. Otherwise AC couple and bias to VCCA/2 for AC signals.

Side notes

• If the signal has a long path, you may also need to consider guarding, shielding and a differential to single ended mixer that combines level shifting and range (gain) reduction if necessary.
• Understand that if any signal driving a CMOS chip goes outside the supply or ground rails or the spec absolute max range can cause SCR latch-up effect, which can cause device failure ( except special cases )
• Clamp diodes give limited low current internal protection.
• Many sensors are designed for unipolar ADC's like this one. What signals are you measuring?
• Also do you have an anti-alias LP filter for your signals if they contain any high frequency?
• When in doubt RTFM... Read The Fine Manual....

Answer 3

No.

However, if you do not need the DC component of the signal, you could capacitively couple it into an ADC input which is biased to the middle of the allowable range, choosing the capacitor and bias resistor values to keep the variation in the allowable range. This will of course have a high-pass filter effect, which you can model and decide if is problematic for your measurement - to a degree you could even compensate for it mathematically.

If you do need the DC component, consider using an op amp to add an offset voltage and adjust the gain as needed to remain within the allowable range.