TXB0108: ADC values reading inaccurately after passing through TXB0108

William Thompson44

Part Number: TXB0108
Other Parts Discussed in Thread: ISO7842, SN74LVC2T45

Hello,

I am passing a MISO line from an 16bit res adc (LTC2326) through the the TXB0108 in order to transition from 5V to 3.3V. I have slowed down sampling to 1per second so its not a speed issue.

Unfortunately the values that I read are all over the place and inaccurate. When I read the values prior to the going through the TXB0108 I read correct values. I have even put the 5V signal through a resistor divider and even that works. It has to be something with the TXB0108.

Am I missing something here? OE is held at 3.3V.

Thanks so much,

Will

over 5 years ago

0 Chad Crosby over 5 years ago

TI__Expert 6680 points

Hi William,

Could you post a picture of your schematic for the circuit?

Thanks!

Chad Crosby

0 William Thompson44 over 5 years ago in reply to Chad Crosby

Prodigy 195 points

I am attaching the entire signal chain, from adc >ISO chip > TXB0108

Like I said before, I get correct values on a test point I have directly before the TXB0108

0 Clemens Ladisch over 5 years ago in reply to William Thompson44

Guru 316860 points

Please show everything connected to the SPIO_MISO3.3 net. How long is this trace?

0 William Thompson44 over 5 years ago in reply to Clemens Ladisch

Prodigy 195 points

The only thing connected is a testpoint and an edge connector gold immersed finger (havnt even connected the edge connector into our main control board yet, just testing off the testpoint). The lead itself is not that long, probably less than 30mm.

0 Chad Crosby over 5 years ago in reply to William Thompson44

TI__Expert 6680 points

Hi William,

The digital isolator you're using is capable of stepping 5V logic to 3.3V logic, and makes the use of the TXB0108 unnecessary.

Assuming your master is using 3.3V, you can supply 5V to your V_ISO supply, and 3.3V to your V_CC side and completely remove the TXB0108 from your circuit.

Thanks!

Chad Crosby

0 William Thompson44 over 5 years ago in reply to Chad Crosby

Prodigy 195 points

This is not possible in my design as I need the 5V supply to feed other components on the isolated side.

Also no voltage should be applied to V_iso. The voltage is produced solely from VCC within the component.

0 Chad Crosby over 5 years ago in reply to William Thompson44

TI__Expert 6680 points

Hi William,

I see that you have your OE pin tied low in the circuit. Are you making sure to drive this high so the device is enabled (see section 8.3.4 & 8.4 of the DS)?

Also, since the ISO7842 has an internal switching converter, I would make sure to add some decoupling capacitors to the supplies of the TXB0108 to be sure there's no excess supply noise (see section 11 of the DS).

Otherwise, if you're not completely attached to the current voltage translator, I would suggest using the SN74LVC2T45 if you only plan on having 2 I/Os.

Thanks!

Chad Crosby

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TXB0108: ADC values reading inaccurately after passing through TXB0108