INA149 PSRR

Petr Kadanik

Other Parts Discussed in Thread: INA149, LM2990, TPS65130

Hi there,

let's go deep into INA149 :-)

There is defined PSRR (power supply rejection ratio) in the datasheet as 120 dB. Then we can see POSITIVE PSRR vs FREQUENCY graph at page 8.

Plots in figure 16 and 17 cover 10 100 kHz bandwidth. WHAT HAPPENS FOR f>100 kHz?

I've attached my schematics. Please see, I've used AD629 as a placeholder as I've found it in my library :-)

OK - what's my problem?

It looks like power supply noise propagates to the output signal. I use TRACO TEN 5-2422 with 100nF ceramic + 4.7uF elyc caps at output.

INA149 has 100nF ceramic close to +12 and -12V and GND. I can see > 1 MHz noise and spikes in power supply voltage.

Could you please comment PSRR paraameter of INA149? Does > 1 MHz power supply noise propagates to the output signal easy?

What's your recommendation to increase INA149 immunity to power supply noise?

over 9 years ago

0 Pete%20Semig over 9 years ago

TI__Mastermind 25775 points

Hello Petr,

I would expect the ac PSRR performance to continue to decrease at a rate of 20dB/decade. Therefore I would only expect only ~20dB PSRR at 1MHz.

I recommend adding additional 1uF decoupling capacitors near the supply pins as shown in Figure 43 of the INA149 data sheet. In addition, you should consider using low-dropout regulators (LDOs) after your switching regulator to provide a 'cleaner' supply voltage for your analog circuitry. For example, you could use the +/-15V version of your switching regulator (or the TPS65130/1) along with +/-12V LDOs (e.g. LM2990 for the negative rail).

0 Petr Kadanik over 9 years ago in reply to Pete%20Semig

Prodigy 200 points

Pete,
thanks for your response.
I've tried LDO, but 1 MHz noise went thru it.
Would you recommend any particular 1uF cap? Would e.g. tantalum work better? Or any specific parameter ensuring the cap will capture 1-10MHz spikes?

Thanks

Petr

0 Pete%20Semig over 9 years ago in reply to Petr Kadanik

TI__Mastermind 25775 points

Hello Petr,

Perhaps this is a PCB layout issue. Please inspect your PCB to ensure that noisy traces/planes are not near your sensitive analog traces. Concerning decoupling capacitors, here are some good references:

e2e.ti.com/.../the-decoupling-capacitor-is-it-really-necessary

e2e.ti.com/.../bypass-capacitors-yes-but-why

www.ti.com/.../sloa069.pdf

www.ti.com/.../sloa089.pdf

www.ti.com/.../scaa082.pdf

The last link has some good information on pages 13-14. Make sure you place both capacitors as close to the power pins as possible. In addition, place the lower valued capacitor closer to the device than the large capacitor.

Concerning capacitor type, I recommend using a ceramic capacitor (C0G/NP0 or X7R/X5R) in this case due to the frequencies involved. Also make sure that their voltage ratings are double the voltage that they are decoupling.

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INA149 PSRR