# High noise on LT1037 ouput [~200 gain]

Discussion in 'Analog & Mixed-Signal Design' started by Alexander Spaett, Jul 1, 2017.

1. ### Alexander Spaett Thread Starter New Member

Jul 1, 2017
3
0
Hello everyone,

so I'm currently trying to get this working:

Everything is going allright. I only got one issue:
Super high noise on the Output Signal as shown in the following Pictures:

- Input: 20 mV [Peak to Peak] at 30 kHz:

- Input: 20 mV [Peak to Peak] at 110 kHz

The Datasheet of the opamp can be found here: http://cds.linear.com/docs/en/datasheet/100737fbs.pdf

I also added one capacitor between +15 V and GND [ C = 100nF] and the same one between -15 V and GND - nothing changed. Sadly these were the biggest capacitors i had available. If you think increasing the capacitance of those could fix the problem i could still add some of those in parralel.

I really hope someone can help me out with this.

Best regards
Alex

Edit: Fixed pictures not showing.

Last edited: Jul 1, 2017
2. ### crutschow Expert

Mar 14, 2008
20,041
5,638
It appears to be noise on the signal.
Can you show what the input signal looks like?

What is the highest frequency signal of interest?
You always want to roll of the response above that frequency to minimize signal noise.
You can do that with a small capacitor across R1.
You might also want to add an RC low-pass filter at the input.

What is the purpose of the low value input resistor, R4?

3. ### Alexander Spaett Thread Starter New Member

Jul 1, 2017
3
0

- I will make a photo from the the input signals later on
- The highest frequency of interest is 200 kHz [lower possible but minimum max frequency is around 175 kHz]
- Thank you for the advice [Capacitor across R1] - i will try this later on and add the results.
- I also considered this already but wanst sure about it since the noise is almost in the frequency range of the signal - but if adding a small capacitor over R1 does not fix the problem i will give it a try

- The R4 has indeed no purpose. It's not placed on the breadboard either.. I did not realize that it was still in the model - sorry for that.

Best regards
Alex

4. ### Alexander Spaett Thread Starter New Member

Jul 1, 2017
3
0
Seems like I solved it.

The following gave a satisfying result:

Due to the capacitor of 100p as well as the lowpass the total dB-gain is a bit lower as well as a slightly "worse" [lowpass] frequency response of the output. But this is not an issue.

Best regards,
Alex

Nov 23, 2012
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6. ### crutschow Expert

Mar 14, 2008
20,041
5,638
R4 is a rather low value and could load the signal source.
You could raise that to 1k or higher (with corresponding reduction in the capacitor size of course) to minimize the loading.

The 100pF across R1 gives a -3dB corner of about 42kHz which will give a rolloff of about 20dB at 200kHz.
This seems like more than you might want.
20pF will give a ≈200kHz rolloff corner, the same as the input corner frequency.
That will give a total rolloff of about -6dB at 200kHz.