Hi all,
My customer has some questions about OPA197.
They evaluated OPA197 based circuit as below;
When the value of R1 is 22k and when it is 160k, the result which is different from the expected output comes out.
Test conditions
Ambient temperature: 25 ℃
VS(Supply voltage ( V + -V- )) :10.8 V / 24 V / 30 V (single supply)
R1 Resistance value : 22k / 33k / 47k / 62k / 82k / 100k / 160kΩ
Input voltage: 0.20 V / 0.65 V / 1.10 V / 1.55 V / 2.00 V / 2.45 V / 2.90 V / 3.35 V / 3.80 V / 4.25 V / 4.7 V
Expected output value for each applied voltage:
1.0 V / 1.4 V / 1.8 V / 2.2 V / 2.6 V / 3.0 V / 3.4 v / 3.8 V / 4.2 V / 4.6 V / 5.0 V
Formula: Vout ≒0.8889*Vin+0.8222 [V]
When R1 = 22k and the input is 0.2V, the expected output is as low as 20mV with respect to 1V, and the input becomes 0.65V or more almost the expected output value.
When R1 is 100k, are close to expected values from 0.2V to 4.7V.
Also, when R1 is over 160 k, the output value will vary.
Questions;
1. What are the possible causes?
2. What is the appropriate R1 value?
3. We believe that OPA197 is designed to be suitable for power saving operation as an FET structure based on its characteristics.
We believe that it is unnecessary to make the equivalent of making the input resistance of two differentials of the Bi-polar circuit equivalent.
However, this malfunction occurred with the equivalent resistance value trial calculated this time.
As shown in the data sheet, we are incorporating proprietary technologies such as input protection, suppression of leakage current, EMI filter,
phase reversal protection function, temperature protection function ... ....
Is there a possibility that the above phenomenon will occur due to the influence of such unique function?
As my TINA based Simulation result, there is no problem on DC results, but Phase margin is less than 30-deg.
So I suspected oscillation, but the output waveform was normal on the actual circuit.
Best regards,
Toshi