ADC108S022: Searching for ADC108S022CIMTX/NOPB replacement options

Hi Sheng,

The ADS7956SRHBR is a 10-bit, 12-channel, SPI interface. This would be the best recommendation for your requirements.

-Kyle

Hi Kyle R. Stone

Thank you for your reply.

I have also looked up information about the ADS7956SRHBR and initially determined that it meets our requirements. However, I still have a few questions that I hope you can help clarify:

① What are the specific advantages of adding an external PGA (e.g., THS4031) to the ADS7956SRHBR?
Originally, we planned to use the ADC108S022, but since the number of channels to be sampled has increased, we are considering either using two ADC108S022s or a single 10-channel ADC.Our goal is to find a replacement with performance comparable to the ADC108S022, just with two additional channels—primarily for cost considerations.Therefore, considering cost factors, the current design will not include a PGA if the ADS7956SRHBR is used. Without the addition of a PGA, how much impact would there be on the sampling performance compared to the ADC108S022?

② The ADC108S022 only requires 4 SPI lines to connect to the CPU, while the ADS7956 has both SPI and GPIO. Is this GPIO a necessary control signal, or can it be left unconnected without CPU control?Or are there any other pins besides GPIO and SPI that must be controlled by the CPU?

③ Does TI offer a digital temperature sensor similar to the DL18B20? If so, could you please recommend one?

Given the tight development schedule, I would greatly appreciate your help in confirming the above details. Thank you very much.

Hello,

The performance impact really depends on the type of input signal being sample. If it is a DC signal or very slow signal such as temperature, you can likely go without using an amplifier, and it is not strictly required. If the input signal is a dynamic, AC signal, it is best to use an amplifier to help drive the RC input of the ADC at higher sampling rates. If you are getting good performance with your current design, then you likely won't need a separate amplifier. You might just need some adjustments to the input RC values.

The GPIOs don't need to be connected. They are optional, but if unused, I recommend tying them to ground. The CS, SCLK, SDI, and SDO pins are the only pins that need to be connected to the controller for data readback and device configuration.

For temperature sensor recommendation, I recommend reposting this question on the sensors E2E forum so the appropriate team can follow up.

Regards,
Joel

Hi Kyle R. Stone

Thank you for your reply.

I still have a few questions I'd like you to confirm.

All the signal sources we use are DC voltage sources, such as DC power supplies for substrates and converted DC voltage values from temperature sensors, which are all slow-changing signals.The maximum conversion rate of the ADC we use is up to 200 ksps.

① The input side configurations of ADS7956SRHBR and ADC108S022s are different. As shown in the figure, the left side shows ADC108S022s' input with separate track-and-hold modes, while the right side shows ADS7956SRHBR's input without track-and-hold modes. If we replace ADC108S022s with ADS7956SRHBR, do we need to add or reserve additional RC hold circuits on the input side? Our original design simply connects DC voltage (after being divided by kΩ-level resistors) directly to the ADC input pins (CH0~CHn) without any RC hold circuits.

② For ADS7956SRHBR, if the power supply is V(+VA)=3.3V、V(+VBD)=3.3V and V(REF)=2.5V, can the ADC（CH0~CHn） accept 3.3V input voltage? Does the input voltage need to be ≤V(REF)?

③ Regarding the reference voltage, the datasheet mentions using low-noise bandgap references like REF5025 to drive this pin. Would it be acceptable if I use a 2.5V generated from a DCDC digital power supply (after bead + capacitor filtering) as the reference voltage for the REF pin?

Thank you very much.

Hi Kyle R. Stone

Could you please help confirm the issue? Thank you.

HI.Joel Meraz

Thank you for your reply. This is very helpful to me.

① I compared some parameters of ADC108S022CIMTX/NOPB and ADS7956SRHBR, such as INL, DNL, offset error, SINAD, SNR, THD, and SFDR. The parameters of ADS7956SRHBR are still slightly inferior to those of ADC108S022CIMTX/NOPB. Could you please explain the reasons for these differences?

② Are there any specific documents that introduce the impact of parameters such as INL, DNL, offset error, SINAD, SNR, THD, and SFDR on ADC performance?

③ Regarding the reference voltage, due to cost considerations, I would like to use the method shown in the diagram below to provide it.

④Regarding the 100kΩ reference input resistor mentioned in the datasheet, what is its purpose? Should it be connected in series with the reference input pin in the circuit design?

Thank you.

HI.Joel Meraz

Thank you for your reply. This is very helpful to me.

①Regarding the 100kΩ reference input resistor mentioned in the datasheet, what is its purpose? Should it be connected in series with the reference input pin in the circuit design?