ADS1248: Multiplexing query

Foz Hughes

Part Number: ADS1248

Hi there,

I wonder if you can help me understand how to use the multiplexing facility on the ADS124X.

Here's some pseudo code explaining what I'm currently doing:

//Setup the first channel:
Write registers (0x40)
Write four registers (0x03)
Set +ve input to AIN0, -ve input to AIN1 (0x01)
Disable VBIAS (0x00)
Set gain & rate (0x05)
Delay 1ms for MUX write

RDATA (0x12)

loop while DRDY==HIGH

Store byte 1 (0xFF)
Store byte 2 (0xFF)
Store byte 3 (0xFF)

//Setup the second channel:
Write registers (0x40)
Write four registers (0x03)
Set +ve input to AIN4, -ve input to AIN5 (0x25)
Disable VBIAS (0x00)
Set gain & rate (0x05)
Delay 1ms for MUX write

RDATA (0x12)

loop while DRDY==HIGH

Store byte 1 (0xFF)
Store byte 2 (0xFF)
Store byte 3 (0xFF)

I have 2 questions about this. First, is it necessary to send an RDATA for each channel? When START is pulled HIGH does the ADC perform a conversion on the currently selected channel, or on all channels simultaneously?

Secondly, From the datasheet section 10.1.8 it suggests that I could speed up and simplify this by sending the commands needed to change the channel while the data is being read, i.e.

Store byte 1 (0x40)
Store byte 2 (0x00)
Store byte 3 (0x25)

But when I try this I get a zero value out. Have I misunderstood?

Many thanks,

Foz Hughes

over 4 years ago

0 Bryan Lizon86 over 4 years ago

TI__Guru 54426 points

Hi Foz,

You do need to send an RDATA command, or otherwise read out data e.g. issuing NOP, after a conversion completes on each channel. The ADS1248 is not a simultaneous sapling ADC nor does it have a multi-byte buffer in which to store multiple data values, so the flow would be something like: start conversions, wait for DRDY to go low, read data, switch channels, wait for DRDY to go low, read data, switch channels, etc.

Do you have logic analyzer data to show the SPI transactions on all digital lines? That would be helpful to understand what you are seeing and diagnose what is going on. I use a Saleae logic analyzer so if you use the same device you can just send the data file and I can open it on my end, instead of sending still images.

-Bryan

0 Bryan Lizon86 over 4 years ago in reply to Bryan Lizon86

TI__Guru 54426 points

You can also check out this e2e post for some pseudo code that takes advantages of the full-duplex nature of the ADS1248's SPI interface:https://e2e.ti.com/support/data-converters/f/data-converters-forum/246594/ads1248-3-channel-multiplexing-following-example

-Bryan

Data converters

Data converters forum

ADS1248: Multiplexing query