Having trouble using PIC16F1828 to communicate with ADS1255.

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

You misunderstood my earlier comment about spi_xfer(). You changed
all the bit counts to 1, and they should all be set to 8. For hardware SPI,
the compiler promotes your '1' bit count to 8. But you should edit it all
to be correct.

For example, in these lines below, you have it set for 1 bit. They should
all be changed to 8 bits.

demedeiros · Joined: 27 Dec 2013 Posts: 71

PCM,

While looking at Ttelmahs original code, he is sending the number of bytes not bits. What is interesting is that if we look at the data on the logic analyzer, the appropriate amount of bits are being transferred. We will provide a screen shot once Olet96 is here. Also we will change all of them to bits to see if that helps the issue.

Thanks!!

olet96 · Joined: 09 Jul 2014 Posts: 16

I have changed all the

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

What do you get if you connect both AIN0 and AIN1 to the reference voltage ?

The EVM board manual for the ads1256 suggests doing that in this section:

olet96 · Joined: 09 Jul 2014 Posts: 16

That seemed to work much better it didnt go completely to zero but its very very close to zero.

temtronic · Posted: Thu Jul 17, 2014 8:57 am

Trying to get a 'real zero' is mighty hard for anything over 12bits in the real world! You have to have a great ground plane, proper wiring, pcb layout, etc.
In the past, I'd run a 'zero' routine. Simply read channel that was 'zeroed' say 8 times, take the average, and use that as the 'reference zero'. Same technique as a 'tare' function.

hth
jay

demedeiros · Joined: 27 Dec 2013 Posts: 71

That makes sense.

I'm trying to figure out why connecting AIN0 & AIN1 to REF produces a zero output, but connecting them both to GND or +5V does not produce a zero.

If AIN0 & AIN1 are shorted together the differential voltage across them will always be zero, irrespective of the voltage applied to them. Is there something I'm missing here?

temtronic · Posted: Thu Jul 17, 2014 12:16 pm

hmmm... 5V/ 24 bits is about .3microvolts. Mighty small,so I suspect that the IF the jumpers you're using to get AN0 and AN1 to ground or +5 aren't EXACTLY the same length.That alone could cause a submicrovolt to be the voltage you're reading. OR your jumper is picking up a wee bit of EMI (noise from PC, cell,etc.) If read at the 'right' time one input will have a larger voltage that the other and with 24bits you'll see that.
Even the PCB layout and cleanliness will play a part in the readings, that's why I use multiple readings and average them. Accuracy is nice but consistency can be more important depending on what the signal source is and what you have to do with it.

hth
jay

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Ttelmah · Joined: 11 Mar 2010 Posts: 19537

In general, all op-amps have a limited voltage range they can handle, and become less linear as you move towards the ends of this range.

There are a lot of factors:
The gain error (I'd suspect this is built to be minimum at Vref, so this is the point where the output value is 'warranted' to be lowest).
Differential non-linearity.
The actual offset error and drift in this.
Then you have no Vref. The supply is not even remotely suitable as a Vref for a circuit at this sort of accuracy level. The comment in the data sheet says it al:

"A high-quality reference voltage capable of driving the
switched capacitor load presented by the ADS1255/6 is
essential for achieving the best performance. Noise and
drift on the reference degrade overall system
performance. It is especially critical that special care be
given to the circuitry generating the reference voltages and
their layout when operating in the low-noise settings (that
is, with low data rates) to prevent the voltage reference
from limiting performance. See the Applications section for
more details."

Then, I think you are running with the buffer on. Look at the restrictions this imposes.

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

His schematic here shows he's using the LT1461 (2.5v I assume) as the
Vref generator:
http://www.ccsinfo.com/forum/viewtopic.php?t=52447&start=9

olet96 · Joined: 09 Jul 2014 Posts: 16

Im using a LT1461bcs8-2.5#PBF which is a 2.5 volt regulator.

olet96 · Joined: 09 Jul 2014 Posts: 16

This morning I tried removing the long wires I had coming off of the AIN0 and AIN1 inputs and soldered a resistor lead across the inputs and noticed my output went very close to zero, however I'm still getting a random spike in the data which i found very strange. I'm thinking of moving the whole experiment somewhere else in the shop to rule out any possible noise.
Here is a sample of data.

olet96 · Joined: 09 Jul 2014 Posts: 16

Demedeiros and I realized our data that we thought was simply a spike or noise is actually the data going into the negative side since we are displaying the 24-bit data in a 32-bit form the first byte is 00 however if we look at the next byte is an FF which puts the MSB of the 24-bit data a 1 which indicates whether or not the number is a negative or not so if I apply the conversion of Data | 0xFF000000; and display the output in decimal I get a negative around -400 which when I plug that into my formula I end up with 596.05x10^-9 x -400 = -238.42 uvolts.

temtronic · Posted: Fri Jul 18, 2014 11:53 am

Welcome to the World of Analog ! As you're finding out, it is very difficult to measure submillivolt levels !!
It takes a mixture of science and black magic to design a real good PCB suitable for analog.You need to KNOW the EMI in the area and properly shiled against it. Use of chokes, caps,wire traces, shileded cables, etc. ALL play an importnat part in the design and execution of a quality product.

You'll probably spend 2-3 months in the lab and still not have the 'perfect' design. Been there..done that.

Without knowing the details of the project, you might want to consider putting the ADC into the 'sensor head' and communicate digitally. Done that a few times over the past 20 years too.

hth
jay