UCD90320: Need help with on chip ADC interface

IzzY P

Part Number: UCD90320
Other Parts Discussed in Thread: UCD90120,

I have been using TI's UCDXXXX devices for a while now, and this time we are planning to use UCD90320ZWST since it has 32 voltage/current monitoring channels. However I find the ADC channels of this device more confusing that the other ones I have used (UCD90120, etc) in the past.

1> My first question is - in the datasheet it says the ADC's input resistance is 2.5Kohms. Also it says to add at least 200 ohms (Pg 47) in series between Voltage rail and ADC pins if the voltage is being sensed without resistor divider. What is the purpose of this 200 ohms? Is it to limit current into the ADC pins? Isn't the 2.5Kohms already in series?

Datasheet: https://www.ti.com/lit/ds/symlink/ucd90320.pdf?ts=1620933963523&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FUCD90320

Please suggest how should I connect these to the UCD's ADC pins. I have attached an image with 3 cases:
2> So if a voltage Rail is being sensed (with or without resistor divider), do we need add an opamp as a buffer before we connect the sense line to the ADC (Case 1)? I don't think we have to, but just making sure.

3> If I am using an output of a buffered current sense amplifier - INA190A3QDCKRQ1, do I still need to add 200 ohms in series? Or let's say can I use 1k ohms and add a 0.1uF cap to make an RC filter before connecting the sense line to ADC? (Case 2)

4> Also what about the Current monitoring pin of TI's smart switch - TPS259827LNRGER? Can we connect it to the UCD's ADC pin for monitoring (with resistor to gnd for voltage conversion of course), or do I need to add a buffer? (Case 3)

Please advise

Image with case 1, 2 and 3: https://drive.google.com/file/d/15FBS16r8Axo8XtBDs6dfwAGMb6qm5vCx/view?usp=sharing

over 4 years ago

0 yihe over 4 years ago

TI__Guru 66170 points

Hello

Please see my comments below.

Basically, you can treat the ADC the same as UCD90xxx part that used previously. There is no difference from design point view.

1. the 200ohm is to limit current for the ESD protection. The 2.5K Radc resistance is between the pin and the sample capacitor. They are different.

2. No, you do not need a buffer.

3. We recommend 200ohms for the current limitation.

4. No buffer is need. but please be noted that the voltage of the IMON of TPS2598 shall not be over the Vref of the UCD90320.

Regards

Yihe

0 IzzY P over 4 years ago in reply to yihe

Prodigy 170 points

Thanks for the prompt reply!

I think that answers most of my questions. I was mainly concerned about the UCD90320's ADC input resistance (2.5kohm) being much much lower than the UCD90120 (0.5 to 8 Meg ohm, depending upon channel). UCD90120 did recommend to keep source impedance to the ADC at 4k ohm or less, which I believe was to avoid voltage division. In one case I didn't not follow that and the measurement had big error.

First question is why was UCD90120's ADC input resistance so high?

Second, I wanted to make sure I can use a higher resistance value in series from source to ADC, say 10K and a cap as a RC filter.

Please confirm.

0 yihe over 4 years ago in reply to IzzY P

TI__Guru 66170 points

Hello

Please see my comments Below:

1. The 2.5KOhm Radc resistance is between the pin and the sample capacitor. It is not the same impedance defined in the UCD90120. You can ignore Radc 2.5Kohms when designing the external voltage divider circuit since it has no impact on it.

2. Yes, you can use 10K resistor with 10-100nF cap to filer the noise.

Regards

Yihe

0 yihe over 4 years ago in reply to yihe

TI__Guru 66170 points

Hello

We haven't heard from you for a while and assumed that the issue has been solved. Please reply if any help is needed.

Regards

Yihe

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UCD90320: Need help with on chip ADC interface