THS4130: THS4130 Harmonics

Hi Jitendra,

at which frequency? What output signal level? Across what load?

It's normal that the distortion increases with frequency and especially with gain. Increasing the gain by a factor of ten can increase the distortion by a factor of ten as well because the OPAmp has less gain reserve.

Also, your feedback resistors are different what is recommended table 9-1 of datasheet.

A good remedy to decrease the distortion is to split the gain evenly on two OPAmps, or even three, if you interested in the higher frequencies.

Kai

Hi

 1. My operation frequency is between 20KHz to 75KHz with ±10Vpp signal level 

 2.For 20 times  gain please recommend RF and Rg values

3.Since we already done the design is any other way can we reduce harmonics?

4.could you please provide the design for the same?

Thanks and Regards,

Jitendra.

You might try raising your supplies to see if the HD goes down, try +/-15V. Also, you might try reducing the feedback R to 5kohm and inputs 500ohm. Also, what is your load, how are you measuring the harmonics. 

>Ok i will try the above inputs and check the harmonics level

>Currently i am not testing with any load

>in real application differential output will go to another card differential amplifier input  through backplane

>I am using signal analyzer to probe outputs of THD4130 outputs P and N pins

>Please provide harmonic measurement setup for the same if i am doing it in wrong way measurement?

Thanks and Regards,

Jitendra

Normally to measure distortion we would use a spectrum analyzer - one approach to that interface shows up in the new FDA data sheets - this output interface shows a 1kohm differential load but a 50ohm source to the spectrum analyzer which it would like. Also, a quick check of 75kHz 20Vpp slew rate shows 4.7V/usec - that is a significan part of the available 50V/usec - rule of thumb for 10x margin says you should not probably expect much better than -70dBc. 

Hi ,

  I have done below test cases

1. THS4130 configured with unity gain and observed with output with no harmonics

2.When i increase THS4130 gain to two i am seeing 2nd harmonic with -20dBc

If its gain issue i could not see any harmonics with 2 gain setting, please provide  me the inputs is anything i am missing?

Thanks and Regards,

Jitendra.

Hi Jitendra,

Jitendra batta said:

2.When i increase THS4130 gain to two i am seeing 2nd harmonic with -20dBc

What frequency? What output amplitude? What feedback resistors?

Jitendra, please be more specific. We cannot help without facts. Please give us some numbers. Tell us how you drive the inputs. Show us a photo of your setup and some scope plots. Show us your layout. All this is necessary to help.

Generally spoken, don't touch the outputs of OPAmp directly with the scope probe but insert an isolation resistor first.

Also, use such a ground spring:

Kai

Hello Jitendra,

  Were you able to lessen the harmonics in the higher gain configuration?

Thank you,

Sima