THS6182: Amplifier DC offset

Hi Yonatan,

If this offset is not seen at the inputs of the device but is seen at the outputs before the ac coupling of the output, the offset is likely due to the THS6182 itself. In the application section of the datasheet on page 17, output offset for this device as well as how to calculate this offset is described. In this section it highlights what factors affect this offset and can give you insight on how to adjust the circuit to your needs while reducing offset.

Best Regards,

Ignacio

Hi Ignacio,

After looking at sector 17 I have still some things that I don't understand,

Is Vio the measured voltage from the positive pin to the ground?

Iib+ is the current flowing on R27 and C10?

Iib - how can I know its value, is it zero in my design?

Hi Yonatan,

Input offset voltage in amplifiers can be modeled as an ideal DC voltage source connected into the noninverting input. It is in series to the components connected to the non-inverting pin similar to the image in the Offset Voltage Section.

Regarding input bias current in the noninverting pin, yes, we can assume the capacitor is open at DC which leaves input bias current to go through the R27 resistor.

The values of these specifications along with the others in the electrical characteristics table of the datasheet give the range you can expect. This range is due to process variation therefore we can not assume any one of these is zero or even the polarity which is why the the formula given in the section shows ± for each of these specs.

I will leave a link to a TI precision labs video that explains these topics in more detail with circuit examples. The informative video can be found here.

Best Regards,

Ignacio

Hi,

I have looked at the videos yet the numbers I get are not even close to the offset I'm having in my design. 

I tried decreasing the resistors values and yet I have the same offset ,i even change the configuration of the amp to only inverting amplifier(shorted to non-inverting pin to ground)

in all those tries I always got the same offset.

Hi Yonatan,

We are very surprised grounding the non-inverting pin results in no difference at the output. Another potential debug option would be to try using feedback resistors at the recommended values suggested in the datasheet, in this case starting at 1000 ohms and tailoring the rest of the resistors around this Rf value. Section titled "Recommended Feedback and Gain Resistor Values" highlights the importance of having an appropriate Rf resistor for this type of amplifier architecture. Along with this architecture is the potential stability problems caused by the addition of feedback capacitance. Along with smaller feedback resistors would it be possible to try removing the feedback capacitance to see if this makes a difference at the output. 

One final debug attempt would be to try a different unit to see if this offset is still appearing at the output or if it changes at all.

Best Regards,

Ignacio