If you DC-couple the THS4508, you risk that it's output goes to the negative rail (in your case -1.5), which would destroy the DRS4 since it's out of its specs.
To move negative signals into the posive range of 0V to 2.5V (allowed range for DRS4), the only thing I see is AC coupling with properly biasing. I don't understand why you try to do DC coupling, that will not work. Alternatively, you coudl invert your negative signal with a small inverter (usually two passive coils) which would bring a negative signal into a positive range.
Best regards,
Stefan
| Mingxin Liu wrote: |
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Dear Dr. Stefan Ritt,
Thank you so much for your detailed and helpful reply. It’s very clear to me now why the standard DRS4 design uses AC coupling, and I appreciate your practical suggestions on biasing and protection.
I have a follow-up question about the THS4508 amplifier I plan to use at the input stage, if you don’t mind. If I want to make the signal path DC-coupled (both input and output), would it be possible to power the THS4508 with dual supplies, e.g., +3.5 V and -1.5 V, so that the input can accept signals in the range of -1 V to +1 V? In this configuration, could the amplifier still achieve a bandwidth of 700–800 MHz, or is that high bandwidth only guaranteed when using a single +5 V supply?
I would really appreciate your insight on this point.
Thanks again for your help and advice!
Best regards,
Mingxin Liu
| Stefan Ritt wrote: |
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We do not have any DC-coupled design for the DRS4. The reason for that is that for many applications we use SiPMs, which are directly powered through the DRS4 board. Since the bias voltage of ~50V would kill the DRS4, we always go with AC-coupling.
The reason the DRS4 evaluation board cannot fully capture signals below -0.5V is the fact that the board only has a single +5V power supply and no +-5V power rails. Having an additional -5V power rail would make the evaluation board too bulky and it could not be powered over USB.
In principle you could bias the point after the AC coupling capacitor to +5V, then you can capture up to -5V signals. Add a divider by two to go up to -10V. But then the input protection diodes won't work as it, you would need some fast 5V zener diodes which open for signals below -5V to protect the following circuitry.
Stefan
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