Part Number: THS3491DDAEVM
Hello everyone,I recently purchased the THS3491DDA EVM, I tried testing it with the Signal Generator and Oscilloscope, as described in the picture below.For my project, My test input range is 1VPP to 5VPP at 10 MHZ max, and have a smooth, no noise signal with a 5VPP to 25 VPP on output respectively. Suffice to say, I was not getting those results. I read the datasheet and it mentioned that the resistor network(Highlighted below) significantly decreases the gain. Is there a way to surpass that resistor network and what would need to be done to get the results I'm seeking above? Thanks. ~Nate
Section 2.2 of the EVM mentions:
"Remember, the output resistor network is only present to protect the measuring instrument from being over-driven by the device output for high-output signal swings. Desoldering the output matching resistor and replacing it with a single 50-Ω series output resistor that gives only 6-dB output voltage loss at the measuring instrument is easily done"
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In reply to Hasan Babiker31:
Thanks for the quick response Hasan,I'm not quite sure What it means when it says "Desoldering the output matching resistor and replacing it with a 50 Ohm Series output"? Also, could something like the following be done?Where all of the resistors can be removed and a connection, such as the one below, can be made? Would the performance of the amplifier be affected in any way? Currently, the gain of the amplifier is 5 V/V. I'd like for it to maintain that gain and amplify my input signal without distortion.Thanks again.
In reply to Nathan Ivkovic:
What is meant by this is exactly as you highlighted in your diagram. The only difference would be that you would have a 50 ohm resistor where the black line is. The purpose of this 50-ohm resistor (and the one at the non-inverting input of the amplifier) is so that you can impedance match with 50-ohm matched equipment. Doing this will help with any reflection issues you might see in your measurements, however there will be 12dB loss as a result (6dB at the input and 6dB at the output).
Otherwise, you can set the EVM in the configuration you showed (with the black line) and remove the 50 ohm resistor at the non-inverting input.
Thanks again Hasan,Just for clarification, would something like this suffice? In total the Resistor at Non-Inverting Input and the Resistor network is removed.
For a 10MHz signal that will work. If at some point you need to use the device at around 200MHz or greater, you will want to use the 50-ohm resistors for impedance matching.
This is a figure to accompany Hasan's response to indicate how to impedance match to a 50 Ohm source and measuring devices.
Hello Hasan, yes 10MHz is basically the max I'm looking for my design. I'll give it a shot and let you know how it goes.
In reply to SimaJalaleddine:
Hi Sima/Hasan,After doing some consulting, I was advised to not make the changes.Apparently, if the resistors are removed, it might significantly limit the amount of voltage supplied to the board. I.e. you wouldn't be able to supply +/- 15V as specified.The reasons were a bit unclear to me, so I wanted to hear from some experts as to why that might happen. Thanks.
I'm not entirely sure what it is you are referring to here. The resistors at the output of the amplifier shouldn't affect the voltages that are being supplied into the amplifier. Are you referring to the decrease in output swing because of a higher output current (as shown in figure 31)? This is only a concern if you are planning to still use the scope in a 50-ohm setting. I would recommend using a high-Z input impedance on the scope, and also making sure your signal generator is not expecting to see a 50-ohm load.
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