THS4121: The output of THS4121

Hi Cris,

Is the customer applying a DC voltage at the input? If yes, then it is possible to not notice any signal at the output because the input is AC-coupled by the series cap C62 and C65. The other thing is that applying a +/-3mV input results in only a +/-150mV output which is a very small signal difference at the output. You might want to increase the signal to +/-15mV in-order to see any noticeable output signal.

Also, is there a reason for the customer to use 1M-ohm resistor on the feedback - R35 and R46? The output of the circuit would be too noisy with the 1M-ohm resistor. I would recommend using a reasonable value for the feedback of ~10-kohms and then scaling the Rg accordingly to 200-ohms for -50 V/V gain.

Best Regards,

Rohit 

Dear Rohit，

Thanks for your answer very much，There's a very big help to me.
But there are another question: Reference voltage test,At first is 1.65v,After a few tests,it is 0v;
Is there Mutation factor? Tks very much.

Hi Cris,

I don't think the reference voltage should be anywhere close to 0V. What circuit provides the reference voltage at pin 2 in the above diagram? Is there an ac-coupling cap of 0.1uF from this pin to GND?

The multiplication factor for the reference voltage test should be very close to 1. So, if the pin 2 voltage in the above schematic is 1.65V, both the pin 4 and 5 voltages should be very close to this value of 1.65V. This condition assumes that there is no differential input being applied at VI+ and VI- terminals.

Has the customer tried to use the feedback of ~10-kohms and then scaling the Rg accordingly to 200-ohms for -50 V/V gain and noticed any difference in the output swing?

Best Regards,
Rohit

Hi Rohit，
The customer has tried to use the feedback of ~10-kohms and then scaling the Rg accordingly to 200-ohms for -50 V/V gain
but has no difference in the output swing，The 1.65v reference voltage has a 100nf filter capacitor，Is this a including factor？
Tks so much.

Hi Cris,

No, the 100nF is not an including factor and is only there to stabilize the 1.65V reference voltage.

Could you please reply to below questions:
1. Is the customer measuring a differential voltage at the THS4121 output pins or single-ended voltage at each outputs?
2. If they are measuring single-ended output voltage, then what value are they measuring for each of the single-ended output voltages?
3. Also, is the customer applying a differential DC voltage at the inputs VI+ and VI-? It is important to note that any DC voltage applied at the input will be blocked at the output by the series cap C62 and C65. So, there will not be any output swing at the THS4121.
4. What circuit provides the reference voltage at pin 2 in the above diagram?

Best Regards,
Rohit