LM6172: Phase Margin for I/V Converter

Sadanori Kato

Guru 36540 points

Part Number: LM6172
Other Parts Discussed in Thread: TINA-TI, PCM1796, PCM1795

Hi,

Could you please tell me how to simulate the phase margin with TINA-TI when using LM6172 as I/V converter ?

LM6172_I_V_Converter.TSC

Best regards,
Kato

over 6 years ago

0 Michael Steffes over 6 years ago

Guru 48395 points

Before starting, you should fix a couple of things needed -

1. Stability in I/V is strongly influenced by the source capacitance - you show nothing, results will be invalid without it

2. 100pF load will hurt phase margin by itself, you should isolate that with an output R before working on phase margin.

0 Michael Steffes over 6 years ago in reply to Michael Steffes

Guru 48395 points

I did also look at the LM6172, that is a high transconductance input stage - gives great slew rate, but relatively poor noise - normally you would use a decomp low noise VFA, but you might have a special situation.

0 Sadanori Kato over 6 years ago in reply to Michael Steffes

Guru 36540 points

Hi Michael-san,

Thank you for your quick response.

1. How should the source impedance be added if using LM6172 for the audio dac such as PCM1795, PCM1796 and so on ?

2. Is the LM6172 damaged due to oscillation if the isolated resistor Riso isn't added to the output node ?

Please tell me how to simulate the phase margin for I/V converter as reference.

Best regards,
Kato

0 Michael Steffes over 6 years ago in reply to Sadanori Kato

Guru 48395 points

Ah yes, a differential output DAC, the datasheet shows some info.

1. The big thing is need to use both sides of the output current signal to suppress HD2 - if single ended, your HD2 will be very poor.

2. Do you need to get single ended? if not just stay diff out from the two I/V stages.

3. getting clean (low HD) diff to single in audio is not easy, lets make sure you need that first.

Two snips from DAC data sheet, these are quite old, surely some newer audio amps could be used

And then more details, but no real design flow to set the I/V feedback C.

The key is the Audio Dac output C, and layout C,

Datasheet does not specify that, so maybe adding it externally would allow a design?

Anyway, for phase margin, this article gives you the correct approach,

0676.The SSU part 5 Stability issues for high speed op amps_introductory issues article text Feb. 2_2019.pdf

And then this article uses this to test and improve phase margin for several decomp apps, concluding with I/V.

6470.Applying High Speed DeCompensated VFAs July1_2019.pdf

0 Sadanori Kato over 6 years ago in reply to Michael Steffes

Guru 36540 points

Hi Michael-san,

Thank you for the information.

I will check these documents, so I will contact you if I have additional questions.

Best regards,
Kato

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LM6172: Phase Margin for I/V Converter