ADS7955-Q1: Interfacing ADC circuitry

Ueliton Rodrigues

Part Number: ADS7955-Q1
Other Parts Discussed in Thread: ADS7955, REF5025

Hi there,

I selected ADS7955-Q1 for my application due to cost and temperature grade of 125C. For this ADC, my application consists of measure 6 battery cells (ranging from 10.5V to 15V) thru a voltage divider and input it to the ADS7955, that talks via SPI with MCU.

I based my circuitry in the ADS7955 datasheet and 'ADS79xxEVM and ADS79xxEVM-PDK User's Guide' (http://www.ti.com/lit/ug/slau262d/slau262d.pdf)

Questions:

1. My Vcc is 3V3. Vref for ADS could be 2.5V? (I'm using REF5025)

2. The 33R is to keep input impedance bellow 50Ohm recommended in the Datasheet?

3. This is my input circuitry. Is that ok?

Regards,

Ueliton

over 5 years ago

0 Cynthia over 5 years ago

TI__Mastermind 24940 points

Hello,

1. Correct, Vref still needs to be 2.5V even though the Vcc is 3.3V
2. The RC filter you see in the in user guide is to be paired with a driving amplifier, not for the impedance. The RC values should be changed to fit the applications needs, and do not need to be used if your application does not call for it.

Regards,
Cynthia

0 Ueliton Rodrigues over 5 years ago in reply to Cynthia

Prodigy 65 points

Thank you very much Cynthia. I appreciate that.

I'm worried about the input impedance of the ADC, so I decided to add a buffer between voltage divider and ADC input to keep the impedance below 50 ohm (mentioned in ADS7955 datasheet, http://www.ti.com/lit/ds/symlink/ads7955.pdf)

Now, my ADC interface is like that:

Do you have any considerations about ADC source impedance?

Regards,

Ueliton Rodrigues

0 Cynthia over 5 years ago in reply to Ueliton Rodrigues

TI__Mastermind 24940 points

What is the sampling rate you are using? Since you are measuring battery voltage, the sampling rate should be rather low, I am assuming.
The 50 ohm impedance is needed when you are running the device at full throughput, but given your use case this is likely not be a concern.
Also, based on the sampling rate, you likely will not need the op amp nor the 33 ohm resistor, and only use the resistor dividor and the capacitor. I think this will likely be the case for your application

Regards, Cynthia

0 Ueliton Rodrigues over 5 years ago in reply to Cynthia

Prodigy 65 points

Thank you Cynthia.

it was very clarifying.

BR,

Ueliton

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ADS7955-Q1: Interfacing ADC circuitry