Part Number: THS3491
Hello,
I am currently debugging a board using the THS3491. The circuit was designed as an inverting amplifier with a gain of -1. The feedback resistor is 681ohm, and the input resistor is also 681ohm.
The intended operation is to generate an output signal equal to a DC level plus a small AC sine-wave component. Specifically, I would like to input a DC voltage from 0.5 V to 2.0 V and add a sine-wave signal from DC to 100 MHz, with approximately 30 mVpp output amplitude.
My current test condition is as follows:
Device: THS3491
Configuration: Inverting gain = -1
Rf = 681ohm
Rin = 681ohm
Non-inverting input, IN+: 0.8 V DC bias
Inverting input, IN-: 100 MHz sine wave, 100 mVpp, AC-coupled through a 10 µF capacitor
Power supply: initially +15 V / -15 V
PD pin: enabled
REF pin: connected to GND
Target output: 0.8 V DC with a 100 MHz sine-wave component, approximately 30 mVpp
When I apply +15 V and -15 V supplies, the output shows severe oscillation or high-frequency noise. However, when I change the negative supply from -15 V to approximately -2.8 V, the output becomes clean and behaves as expected. In this condition, I can observe approximately 0.8 V DC plus a 100 MHz sine wave of about 30 mVpp.
However, when I increase the IN+ DC bias from 0.8 V to about 1.1 V, oscillation or noise appears again at the output. If I then change the negative supply to approximately -2.4 V, the output becomes clean again.
It seems that the stability strongly depends on the combination of the negative supply voltage and the DC bias voltage at IN+. I would like to understand the root cause of this behavior.
Could you please help review the following points?
Is Rf = 681ohm appropriate for a THS3491 inverting gain of -1 configuration?
Since this is a current-feedback amplifier, I understand that the absolute value of Rf is important for stability.
For an inverting gain of -1 configuration, what Rf and Rin values do you recommend for stable operation?
Could the oscillation be caused by insufficient phase margin due to feedback resistor value, PCB parasitics, output loading, or capacitive loading?
Is it expected that changing the negative supply voltage changes the stability behavior in this way?
Is the THS3491 suitable for my target application, which is to output a 0.5 V to 2.0 V DC level with a small 0 to 100 MHz sine-wave component of approximately 30 mVpp?
If THS3491 is not the best fit, could you recommend a more suitable device for this application?
I am looking for a high-speed amplifier or summing amplifier that can operate with a low DC output level of 0.5 V to 2.0 V and superimpose a small sine-wave signal up to 100 MHz.
Additional notes:
The AC input is currently coupled through a 10 µF capacitor.
The output amplitude is small, approximately 30 mVpp.
The load condition can be adjusted if needed.
I would prefer a device that is easier to stabilize than the THS3491, if high output current is not required.
Please let me know if you need the schematic, PCB layout, output waveform, or supply noise waveform for further review.
Best regards,
JH LEE
