OPA2228: Not receiving expected output

Kimberly Acosta

Part Number: OPA2228
Other Parts Discussed in Thread: OPA228

Hello,

This is a follow up from E2E link OPAMP-NOISECALC: SO#447725 - Not receiving expected output - Amplifiers forum - Amplifiers - TI E2E support forums .

Please help our team with the response for this inquiry as customer is chasing us for an update.

7 months ago

0 Art Kay 7 months ago

TI__Mastermind 29258 points

Kimberly,

Here is the original post for completeness:

Customer comments below:
I provided a input of 0.2V AC signal to the non-inverting pin of the opamp and making a 5x gain in inverting ckt and getting 1V at the output of the opamp. I have changed the frequency of the input signal and read the output signal of the opamp .
I have checked different frequencies from 1HZ to 10MHZ , but It is giving correct data upto 1.5 MHZ after that distortion started. The IC OPA2228UA has bandwidth 33MHZ as mentioned in the datasheet. SO we are not getting the output as mentioned in the datasheet . Please do the needful action. I attached a video as proof of data and gave an image of the product.

The OPA2228 is a decompensated amplifier and requires a minimum gain of 5V/V to function correctly. You are using 5V/V, so no issues with the gain setting.
The gain bandwidth product for OPA2228 is 33MHz. This can be used to find the closed loop bandwidth: BW = GBP/G = 33MHz/5 = 6.6MHz. This is the closed loop small signal bandwidth of the device in a gain of 5V/V. If you sweep a small signal sine wave input, you should not see any attenuation of distortion up to 6.6MHz. Above 6.6MHz the signal will be attenuated. Small signal generally means 100mV or less.
I think the issue you are seeing is slew induced distortion. This is related to the slew-rate of the op amp and the peak-to-peak signal expected at the output. The maximum peak-to-peak signal without slew-induced distortion is given by Vpp = SR/(π·f). This function is graphed and shown in figure 6-30. If you are below the curve shown in 6-30 you will have minimal distortion. If you are above the curve you will see noticeable slew-induced distortion on an oscilloscope. For the OPA228 the peak-to-peak output signal is significantly limited above 1MHz. At 1.5MHz, the maximum peak-to-peak signal is given by: Vpp = (10V/us)/(π·1.5MHz) = 2.12Vpp (or 1.06Vpk). I think you are seeing slew induced distortion. You need to choose an amplifier with higher slew-rate.