Possible issue with a simple op-amp amplifier for ADC application.

[ItsMike]

Hey everyone.

As part of a project I need to be able to read a motor's current and the power supply's voltage.

I wanted to amplify the voltage drop on the current sense resistor (low side sensing) with an MC3303 op amp.

I chose this op amp because of it's high supply voltage. I need to power it with 28-32v.

Since I want to feed the voltage to an MCU's ADC I need the max voltage to be 5v or 1.1v (internal vref).

Now what I just realized is that the op amp is not rail to rail so even if the current through the motor is 0 the op amp will not output 0v.

How much will it output when the input is 0v ? and the supply voltage is 28 ? Can I get that info from the datasheet somehow ?

How can I overcome this and have a linear response ?

 

[alec_t]

How much will it output when the input is 0v ? and the supply voltage is 28 ? Can I get that info from the datasheet somehow ?

I would expect the datasheet from a reliable manufacturer to include that info (I don't have the datasheet). Check the max and min ratings.

 

[ericgibbs]

hi,
Look at the d/s extract, states it can swing to ground with a single supply and to within ~2V of the +V supply.

What is the lowest output voltage you are measuring ? and what is the load?

 

[ItsMike]

Anyways, from looking at different manufacturers datasheet i realized that this op amp is exactly what i need.

The MC3403 is a low cost, quad operational amplifier with true differential inputs. The device has electrical characteristics similar to the popular MC1741C. However, the MC3403 has several distinct advantages over standard operational amplifier types in single supply applications. The quad amplifier can operate at supply voltages as low
as 3.0 V or as high as 36 V with quiescent currents about one third of those associated with the MC1741C (on a per amplifier basis). The common mode input range includes the negative supply, thereby eliminating the necessity for external biasing components in many applications. The output voltage range also includes the negative power supply voltage

I'm going to measure the current through a motor using a 0.1ohm resistor.
And the voltage drop on that resistor is going to be about 100mv (1A) and about 140mv (1.4A) peak.
I would also need to detect that the motor is not running by sensing current, the lower I could sense the better.

Now that I look deeper in the datasheet, I see the input offset voltage is 1-8mV. Could that be a problem when amplifying small voltages ? (5mv or so ?)
Can I fix the offset by using software calibration ?
Error= ADC result when (V=0).
True result= ADC result - error ?

What can I do if the input offset voltage is negative ? I don't feel like using an ADC with two voltage references.

 

[ericgibbs]

hi Mike,
The only difference between the MC3303 and MC3403 is its operating temperature range, the clip you posted from the d/s applies to both the 3303 and 3403.

If you post your circuit perhaps we can suggest ways to null out the offset voltage, as you say, a software solution is possible.