DAC8742HEVM: DAC8742HEVM - doesn't modulate a signal

Hi Anastasia,

In the higher current case (2.8mA) what is the amplitude of the HART signal above 12mV? Does the attenuation 'level off' like it is only happening it 4mA or is it continuous.

For the higher current case (2.8mA supply, 4mA loop) can you provide waveforms of MODOUT on DAC874xH, C2 on DAC161S997, C3 of DAC161S997, and LOOP-? This will help show where this signal attenuation is occurring.

Also, what happens if they reduce the current to a value between 2.8mA and 1.6mA? Does the signal become stable?

Did they see this problem using just the EVMs with the EVM GUIs running on separate computers, the setup I described previously?

Thanks,
Garrett

Hello, Garrett!

Thank you for a fast reply! I'll start with your last question.

The problem described above takes place on their own board. All connections are done with your previous recommendations.. So I'll ask them to send me wafeforms and forward it to you.

BEst regards, Anastasia

Hello, Garrett!

I attach waveforms.

Those pictures named as  "4 mA" and "5 mA" mean that it is a current assigned in current loop.

With 12 mA and with 5 mA the Hart signal amplitudes are the same. That's why they made waveforms only with  5 mA.

But here the current consumption is nearly  2 mA. 

Do you need an extra waveforms? Can you download a .zip file?

Best regards, Anastasiadac_osc2.zip

Hi Anastasia,

I looked at your waveforms and in both the "4mA" and "5mA" cases the MODOUT waveform looks like it is unstable. It should be consistent amplitude but in the captures you provided it seems to shift downward. Also, the loop current HART signal looks very noisy and inconsistent which may be due to the issue with MODOUT. When I connected the EVMs I was able to get a clean HART signal onto the loop as you can see in my previous post. Did they switch R11 out for a 500 ohm resistor and switch the capacitors as I described?

I also noticed they appear to be using a different board than the orderable EVM based on the picture in your previous post. Is this correct? This looks like a pre-release version and I don't have the schematic on hand. The schematic and the jumper settings in the EVM User's Guide will not be correct for this board. I would recommend they use the final version of the EVM which I have linked to below.

http://www.ti.com/tool/DAC8742HEVM

Thanks,

Garrett

Hello, Garrett!

We've ordered exactly DAC8742HEVM for customer. But on their own board they use XDAC8740HRGET ICs (also ordered as samples).

I'll detail information about resistance and capacitance.

Best regards, Anastasia

Garrett,

They've made all changes according to your recommendations ( R11 changed to 499 Ohm, high capacitances on current loop input are put away, input DAC capacitances are swapped (С1 pin 14 and С3 pin 12).

Maybe, smth wrong?

Best regards, Anastasia

Hi Anastasia,

Understood, I was just checking because the board photo in your previous post is definitely not the final version of the DAC8742H EVM.

I wanted to ensure that the HART output amplitude does not change when the DC current is at 4mA compared to higher values. I set these boards up as I described in my previous post and took scope captures of the output and the HART input signal from DAC8742H. I confirmed that with how I have connected these two EVMs the output HART amplitude does not change with DAC161S997 DC current value.

The first capture is with the loop current set to 10mA. The HART signal is the red waveform measured across a ~250ohm resistor.

The second waveform is with the same HART signal input but the loop current set to 4mA. You can see that the amplitude is around 250mV in both cases.

The third waveform is the HART input signal from DAC8742H measured at MODOUT while connected to DAC161S997 EVM (500mV pk-pk).

You can also see that the HART input signal and the loop HART signal are clean waveforms. Even if the loop HART signal has an issue I would expect that the waveform would be fine on MODOUT. In the scope captures you shared the MODOUT signal was not what I would expect and the loop HART signal did not have a constant amplitude in ether the '4mA' case or '5mA' case. There is an issue with the MODOUT signal. Can they provide a capture of MODOUT when disconnected from DAC161S997?

Thanks,

Garrett

Hi Anastasia,

Do you have any update on this thread?

Thanks,
Garrett

Hello, Garrett!

One of customer's engineer was on vacation last week. So I'm waiting an update and I'll write to you ASAP :)

Best regards, Anastasia

Garrett, hello,

So..

I attach customer's schematic (the striked out components are not mounted on board).

Earlier they've done a mistake, the capacitors  (pin.С1, pin.С2 of DAC) were already mounted on their board according to TI's recommendations. With such configuration the HART signal in current loop was absent.

They've done some modification (0.39uF to  С1 of DAC, 1000 pF on pin С3 of DAC). And the HART signal in current loop appeared (the waveforms were sent earlier).

So does it mean that the capacitors on  DAC161S997 EVM are connected to DAC's pins right?
I attach also waveforms from modem output (Hart out) with DAC disconnected (waveform №2,3 are scoped in  AC and DC modes).

The schematic of this board (from which waveforms are scoped) is also in attachment.

The waveforms are the same with DAC connected. The waveforms are scoped with loop current 12 mA.

We could see on the waveform 1 the following: the board receives Hart frame from external modem (Metran), the frame is right. Then the board should send a reply. The reply frame is created but we could see only single burst (impulse).

On customer's schematic the  С31 is  0,33 uF (they tried to increase it to  0,39uF, but the waveforms are the same).

hart_osc.zip

Best regards, Anastasiaivd_4_pcb_stm_v2_.pdf

hi, Garrett!

Thanks a lot for your invaluable help. So waiting for results of your experiment.

Best regards, Anastasia

Hello, Garrett.

We need to clarify some moments. Earlier you suggested to make some modifications with  DAC161S997 EVM (maybe we undersood smth wrong):

1. To swap around capasitors C20 and C17 (you said that there is an error in PCB layout - in place where these capacitors are switching). Also you said this fact heavily attenuates HART signal before gain stage. That's why we can't see no HART signal on output.

There are  С1 = 390 nF and С3 = 1 nF on EVM. And if we swap them one to each other we'll get a scheme from my customer.

But now you say that the schematic of EVM is right ? So should we swap them or not (or meybe we understand smth wrong)?

They'll try to take away С22, L2 and L3 and look at waveforms.

Best regards, Anastasia.

Hello, Garrett! Now I understand. Thank's a lot. Do you already have some results from previous post?

Best regards, Anastasia

Hi Anastasia,

I was suggesting that they try these modifications to their board. In my previous tests, I used the EVMs for both devices but they have developed their own PCB. Did they ever get the correct waveforms using the two EVMs and the instructions I provided?

Thanks,
Garrett

Hi, Garrett.

I'm clearing it up...because all last communication with them was about their own board.

I'll write you as soon as I know it :)

Best regards, Anastasia

Hi Anastasia,

Do you have any update on this thread?

Thanks,
Garrett

HEllo, Garrett.

Unfortunately I was on vacation for a few days. I'll try to refresh the information about our thread. Please don't close it.

Best regards, Anastasia

Hi Anastasia,

When do you expect to have more test results?

Thanks,
Garrett

Hello Anastasia,

Do you have any news for Garrett on this topic?

Hello, Duke.

Unfortunately, the customer's keeping silent...

I suppose you could close this thread....I'm sorry about annoying you. I think they've solved problem.

Thank you for great help. If smth goes wrong I'll write to forum again.

Best regards, Anastasia

Hi Anastasia,

I will go ahead and close this thread but we are happy to provide support. If they have any other issues please use the ask a related question button to start a new thread.

Thanks,
Garrett

Hi, Garrett!

Thank's a lot for your help.

Have a nice day)

Best regards, Anastasia