Thank you for considering the ADHV4702 on your design.
Based on your results, I think the amplifier is being limited by the Slew Rate.
If you're using the Evaluation board in its default configuration (Aside from the gain network that you changed to 40V/V), the input clamping diodes installed using SBAV199LT1G limits the slew boosting circuit feature of the ADHV4702 as indicated on the User Guide's input clamping section:
Then looking at the datasheet spec for Slew Rate:
The external input clamping diodes limits the slew rate to 24V/us, given by the formula:
Slew Rate = Vo x 2πf; where Vo is the voltage peak, and f is the full power bandwidth;
With your Vo = 50Vpk and Slew Rate = 24V/us;
The part can only take an input of around 76kHz. thus going above 70kHz will introduce some attenuation on the output.
You can try to remove the input clamping diode but I'm afraid this can only accommodate to around 235kHz at your specified Vpeak of 50V or Vpp of 100V.
I hope this helps Zenlu.
Let me know if this clears up your concern.
Hi, I am using ADHV 4702 evaluation board. but the output is too low when frequency going up to 70KHz. below is the list of the output and frequency measurements. the gain network is 40V/V. input is Vpp=4V. I am using the onboard +110/-110 power supply.
My design goal is 250KHz, output Vpp=100V. Currently, only work below 60KHz, can output 100V be reached.
this design is our own, not a direct recommendation for the ADHV4702-1. We ordered two EvalKits now for the LT8365 (one for -HV Application and one for +HV-Application) to check if the sporadic design is provoced by the dimensions of external components or by layout. Also damage during the soldering process or thermal issues in operation could be possible. Or maybe the components were flawed by the distributor already, but I think thit is unlikely. We will see and have contacted now an employee of Analog Devices. Thank you anyway.
Hi Chrisbo, I'm not familiar with the LT8365. To clarify, is this possibly the design that is used on the Eval Board, or is the power supply a design of your own? I believe the Power group would be the right group to address this, unless it's the Power Supply that is recommended as a reference design for the ADHV4702-1. Please let me know. Best Rgds, Brian Beucler