ADS8028: understanding conversion wrt datasheet

Matt Mishrikey

Part Number: ADS8028

I think I am completing successful conversion using the ads8028, but there are some nagging questions that make me uncertain:

In my test case I am reading in a 200mV reference on channel 7 (last channel of 8 channel ADC). Internal reference of 2.5V is being used.

First step is to write the conversion command. In my case this is two bits that are high in the first 16 bit frame. The first bit is the write command, the second bit specifies the seventh channel AIN7.

The next frame is the conversion frame, so data is ignored.

The following frame (third frame) is when I should get my data for the first request.

I think I am getting my data however in the fourth frame. The fourth frame shows 0x7144, or channel seven, decimal 324/4095*2.5 = 0.19780V.

question 1) why is the data in the fourth frame, and not in the third as the data sheet indicates?

question 2) what is in the third frame? garbage? 6FFF indicates 2.5V on channel 6 which I never requested.

Thanks

over 3 years ago

0 Matt Mishrikey over 3 years ago

Intellectual 445 points

Here is a screenshot from the datasheet.

0 Cynthia over 3 years ago in reply to Matt Mishrikey

TI__Mastermind 24940 points

Hello,

This device uses CS falling edge as the first falling edge to clock data in or out.

Would you try changing the idle state of SCLK from low to high. This will have SCLK high when CS has a falling edge, and then first SCLK falling edge will output ADD2 on SDO. you still need 16 falling SCLK edges.

Let me know how that works

Regards

Cynthia.

0 Matt Mishrikey over 3 years ago in reply to Cynthia

Intellectual 445 points

Great thinking.

That got rid of the extraneous data frame. However - the data read out, now on the third frame, is a bit off. Now it shows 0xE288 instead of 0x7144. That's a one bit left shift. Hmm.