AFE5803: What is the voltage level of the SDOUT pin?

Jarrod Eliason

Part Number: AFE5803
Other Parts Discussed in Thread: SN74AUP1T04, , ISO7240M

According to Table 12 of the AFE5803 datasheet, the ADC SPI output is powered from DVDD (1.8V). Following Figure 58, there is also a note saying that the "SN74AUP1T04 can be used to convert 1.8V logic to 2.5V/3.3V..." And, the Digital Characteristics table shows VOH of SDOUT being DVDD (1.8V) typical. This all seems to say that SDOUT is a 1.8V signal.

However, looking at the AFE5803EVM schematic, SDOUT is fed into pin 3 of U23, which is an ISO7240MDW with VCC1 connected to AVDD (3.3V). According to the ISO7240M datasheet, VIH min is 0.7*VCC, which would be 2.3V. If so, how would a 1.8V SDOUT signal be guaranteed to work?

To anyone with the same question, it appears from the datasheet that the SPI input and RESET signals all require 3.3V levels, while only SDOUT is at 1.8V.

10 months ago

0 Shabbir Amjhera Wala 10 months ago

TI__Expert 4491 points

Hi,

You are correct that there are chances that SDOUT read may fail with the scheme shown in AFE5803 EVM. It seems to be a bug in design. In your design it will be better to connect the SDOUT to the IC that accept 1.8V logic level.

Thanks!

Regards,

Shabbir

0 Jarrod Eliason 10 months ago in reply to Shabbir Amjhera Wala

Prodigy 30 points

Thanks for the answer. Is SDOUT always 1.8V, or is it 1.8V when it comes from the ADC registers and 3.3V when it comes from the VCA registers?

0 Shabbir Amjhera Wala 10 months ago in reply to Jarrod Eliason

TI__Expert 4491 points

Hi Jarrod,

That's a good point. I checked in design. In VCA die, SDOUT buffer is working on 3.3V supply. So when SDOUT is driven by VCA die, it will be 3.3V and when driven by ADC die, it will 1.8V. But since the SDOUT of two dies are shorted and the ESD diode on SDOUT buffer of ADC die will clip SDOUT signal level to ~2.5V level.

Thanks!

Regards,

Shabbir

0 Jarrod Eliason 10 months ago in reply to Shabbir Amjhera Wala

Prodigy 30 points

I have to say that this is the first time in my 27 year career that I have had to accommodate a signal that operates in 2 different power domains. It is hinted at in the datasheet, but I wasn't confident in my interpretation until you explicitly stated it here. Thanks for clarifying.

0 Jarrod Eliason 10 months ago in reply to Shabbir Amjhera Wala

Prodigy 30 points

I should also ask, then, about the SPI inputs. Are they also shared between a 3.3V die and a 1.8V die such that they will be clamped? If so, what voltage do you recommend that we use to drive the SDATA, SCLK, SEN, and RESET signals?

0 Shabbir Amjhera Wala 9 months ago in reply to Jarrod Eliason

TI__Expert 4491 points

Hi Jarrod,

You can apply signal with 3.3V level. The same signal goes to 1.8V die but there is no input diode to supply hence it wont clamp it.

Thanks!

Regards,

Shabbir

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AFE5803: What is the voltage level of the SDOUT pin?