ADS 8254 doesn’t show correct data

Vladimir Puzakov

Other Parts Discussed in Thread: OMAP-L137, ADS8284

We are evaluating ADS 8254 for our application. We have connected the ADC to OMAP-L137 EVM board. After generating proper sequence of control signals we can’t get the proper information on DATA bus. We tested all parts of the ADC (reference, analog buffers, multiplexor and all data-flow control signals like CS, RD, BYTE) and it work properly. We are trying to capture analog signal on CH0 in manual mode with AUTO=0, C3…C1=0, RDn=0, SCn=0, BYTE=0. We are setting CONVSTn low for 2us and getting BUSY high for 650ns but all bits DB0..DB15 stay always in high. We can’t get any result different from 0xFFFF. The data bus is disconnected from all others ICs and driven by ADC only. Could you explain this situation?

Thank you,

Vladimir.

over 14 years ago

0 Tom Hendrick over 14 years ago

TI__Guru**** 173026 points

Hi Vladimir,

Could you possibly post a schematic of the connections between your L137 and the ADS8254? I'm sure your control signals are fine, but if you can post a logic analyzer plot or O-scope screen capture showing the timings, we might be able to try and reproduce your issues here to see what might be going on.

0 Vladimir Puzakov over 14 years ago in reply to Tom Hendrick

Prodigy 80 points

There is the part of the schematic related to ADC

And there is the wave form CH1- CONVSTn, CH2 - BUSY

The rest of ADC control signals are low ( AUTO, C1-C3, CSn, RDn, BYTE, MXCLK) to simplify the situation.

The analog signal is applied to CH0 with 2V common mode level. Multiplexer and preamplifier work correctly.

We have tree ADCs with absolutely same behavior – data bus is always 0xFFFF.

Thank you,

Vladimir.

0 Tom Hendrick over 14 years ago in reply to Vladimir Puzakov

TI__Guru**** 173026 points

Thank you Vladimir,

Please try tying BUSY back around to MXCLK - the MXCLK needs to toggle regardless of manual or auto channel selection (see page 10 - Multiplexer Control Pins), also, your schematic appears to have the CHxP and CHxM pins shorted together. Is this the case or is it just an anomaly of the screen shot? If they are shorted, that gives you a voltage differential of 0V between the terminals which could be causing the 0xFFFF output data (see Table 3 page 29).

0 Vladimir Puzakov over 14 years ago in reply to Tom Hendrick

Prodigy 80 points

We have tried this connection. It doesn't help. And also I have sad that multiplexer is working properly because I see the INP and ANM signals according to input signal.

You right about the schematic. There is the proper view of it

Regards,

Vladimir.

0 Vladimir Puzakov over 14 years ago in reply to Vladimir Puzakov

Prodigy 80 points

Hi Tom,

Any other suggestions why ADS8524 doesn't work?

Regards,

Vladimir.

0 Tom Hendrick over 14 years ago in reply to Vladimir Puzakov

TI__Guru**** 173026 points

Hi Vladimir,

At the moment, no. I've been over your schematic again and don't see any obvious problems. Holding the /CS and /RD lines low should allow the device to run in 'Auto Read' mode, so I don't see any problem there either. I'll send you an e-mail and we'll see if we can't get a little more detail on whats going on with your hardware.

0 vervs over 13 years ago in reply to Vladimir Puzakov

Prodigy 20 points

Hi Vladimir,

I have the same problem, the difference is that uses the 8 bit data bus and CS pin tied to GND (Figure 4, p.14). Could you tell what was the reason in you case?

Thank you!

0 Vladimir Puzakov over 13 years ago in reply to vervs

Prodigy 80 points

Hi,

In our case, we had problem with the chip itself. We received free samples from TI that I soldered on prototyping board. So the chip was probably damaged by heat.

Then I got evaluation board which was working properly.

Regards,

Vladimir.

0 vervs over 13 years ago in reply to Vladimir Puzakov

Prodigy 20 points

Hi Vladimir,

Thank you for your reply.

I also used the samples, possibly I have the same problem. I could not find the evaluation board for ADS8254, could you tell about your board?

0 Vladimir Puzakov over 13 years ago in reply to vervs

Prodigy 80 points

Hi,

I got pre-production ADS8284 EVM from Tom Hendrick

Regards,

Vladimir.

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ADS 8254 doesn’t show correct data