TLV9001: PWM DAC -> Sallen Key Butterworth Low Pass Filter

d_zero

Genius 4470 points

Part Number: TLV9001
Other Parts Discussed in Thread: TLV9061, TLV9062,

Hi All,

i'm trying to design a PWM DAC with Active Second order Sallen Key Butterworth Low Pass Filter with one stage.

But i got spikes on the Vo please see simulation file attached.

Here is my calculation as per: SLOA049B

Sallen-Key
	R1	2.321663	kOhm	Butterworth
	R2	10.18273	kOhm	FSF	1
	C1	10	nF	Q	0.707107
	C2	33.06982	nF



	f_PWM	100	kHz
	f_c	1.8	kHz

	n	3.306982
	m	0.228
	SQRT(mn)	0.868327
	R	10.18273	kOhm
	C	10	nF

SallenKey.TSC

Thank you for the help.

Br,

over 2 years ago

0 Pradeep Jacob over 2 years ago

TI__Prodigy 190 points

Hello,

Please allow some time for us to deep dive into why the output is not responding correctly. I will look into this matter and get back to you.

Thank you,

Pradeep Jacob

0 kai klaas69 over 2 years ago

Guru 140200 points

Hi,

it's a known fact that active low pass filters do fail in many cases due to the imperfections of OPAmp at the highest frequencies. Think of errors due to finite output impedance, finite open loop gain, etc.

It's always wise to filter out the highest frequency contents by the help of a passive low pass filter.

Kai

0 kai klaas69 over 2 years ago in reply to kai klaas69

Guru 140200 points

Like this, for instance:

d_tlv9002_1.TSC

Kai

0 d_zero over 2 years ago in reply to kai klaas69

Genius 4470 points

Hi Kai,

thank you for the answer. I also did some research and because of the square wave of the PWM the OPAMP bandwidth can't cover the high freq of the square pulse.

So i tried with TLV9061/TLV9062, and some adjustments ( instead of Butterworth i used Bessel coefficients ), and without the input RC filter i think i made it filter the spikes out so that spike amplitude is comparable to desired output voltage.

Please check my sim.SallenKey_NEW.TSC

By the way, i tried also your approach to add RC filter on the input, but i didn't manage to get it work, how did you choose the values?

Best Regards,
D.

0 Pradeep Jacob over 2 years ago in reply to d_zero

TI__Prodigy 190 points

Hi,

You can use the Filter Design Tool to help pick out the correct values.

Best Regards,

Pradeep Jacob

+1 kai klaas69 over 2 years ago in reply to d_zero

Guru 140200 points

Hi,

yes, the performance looks much better with an OPAmp which is ten times faster:

But even the TLV9001 can show good results when adding a passive low pass filter.

I have chosen the component values by trial and error. But if you want the Bessel curve, the www has lots of examples. I adapted one and came to this circuit: