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DAC8742HEVM: DAC8742HEVM - doesn't modulate a signal

Part Number: DAC8742HEVM
Other Parts Discussed in Thread: DAC161S997, DAC161P997, DAC8742H

Hello, hotline.

Our customer's playing with DAC8742HEVM. 

They try to simulate a modulation on the modem output. But there is no signal on the output line of IC DAC8742 (signal MODOUT, the testpoint on EVM for oscilloscope probe is TP13).

They use  TI GIU for this EVM. The modulation simulation they manage by pushing “Generate Write” button (see the photo).

Modem configuration is: Internal reference; internal filter mode; hart interface, internal oscillator, UART interface (connecting with MCU).

What do they do wrong? Why is there no modulation?

Best regards, Anastasia

  • Hi Anastasia,

    Based on the GUI photo you provided, the internal reference does not appear to be enabled. The internal reference must be enabled by clicking the 'precision reference' block in the block diagram.

  • Hi, Garrett!

    They've activeted  'precision reference'. And the modulator began to work.

    But there is new problem. They connect DAC8740 EVM and DAC161S997.

    There comes an error when they connect EVM DAC161S997 with DAC161S997 GUI (please look at attached slide). They tried to reinstall GUI (for the first time the program didn't recognize port). After reinstallation the error came.

    Also they connect EVMs according with recommendations to the load  in the current loop (the load is about 500 Ohms). And converter Hart-usb (Metran-682) is connected.

    In DAC8742H GUI they make modulation simulation by pressing “Generate Write” button. There is a signal on the modem output with 250 mV amplitude). There is a signal with 10-15 mV amplitude on an dac161p997 input (pin 13). So it is accordingly to Datasheet ( 15 mV). But there is no modulation signal in load and  Hart-usb converter doesn't fix a modulation in line.


    On the modem side in  Hart Tx line there is filter already made on С28, С23. Maybe some capasitors are to be shorted when we connect these two EVMs? 

    Also when they try to modulate the signal wtih Hart-usb converter there is a signal on Hart_Rx1 line but it is very poor and the  DAC8742H GUI fixes it very unstable (one from 10 frames is received).

    HOw could they check reading function in DAC8742H GUI correctly?

    As they expect when the program DAC161S997 GUI starts working correctly the DAC will activate and everything will work.

    Here below there is photo EVMs connected together (AGND line of  DAC8742H EVM is connected with COM line of DAC161S997 EVM).

    Best regards, Anastasia

  • Hi Anastasia,

    Did you ever get the DAC161S997 EVM functioning before trying to interface it with the HART modem? Connecting the EVMs together should not affect the operation of the DAC161S997 from a software perspective. Does this error occur when launching the EVM software or after some action in the software?

    One way you can check the reading function of the DAC8742H by setting the device to full duplex with MODIN connected to MODOUT. You can then transmit HART by clicking 'generate write' the read back directly by clicking 'generate read'. If you a using UART mode you will have to specify the number of bytes to read.

    Can you show exactly how they are connecting to the HART-USB converter and what converter they are using?


  • Hello, Garrett!

    At the moment I can only answer one question about the converter type. They use 

    I'll refine an information from customer and forward it to you.

    Thank's and Best regards, Anastasia

  • hi, Garrett.

    The problem stil remains. There is no signal on DAC161S997 EVM...

    Could be a problem connected with filters of DAC8740 EVM and DAC161S997 EVM? Maybe some of them should be shorted?

    Maybe you have a working example of connecting these two EVM with step-by-step recommendations?

    If the OS could play any role (they use Windows10)?

    Best regards, Anastasia.

  • Hi Anastasia,

    Were they able to solve the error with the EVM software? It looks like a driver or dependency is missing which could be related to the OS or other software installed. The EVM software is compatible with Windows 10. If they cannot launch the EVM software how are they applying a HART signal to the DAC161S997? Maybe they could try installing on another computer.

    I will try to replicate this by using the DAC874xH EVM to modulate the HART signal with the DAC161S997 EVM and let you know the steps required.

  • Hello, dear Garrett!

    I'll be very grateful if you could try to reproduce customer's case.

    As for software they'll try to reinstall it on the other PC..

    So I'm waiting for the results of your experiment

    Best regards, Anastasia

  • Hi Anastasia,

    I wanted to provide an update on using the DAC874xH EVM with the DAC161S997 EVM. I was able to get this working but it requires a few board modifications.

    First off, the software error you were seeing is caused by trying to launch both EVM's software on the same computer. This error can be solved by simply using two computers, one for each EVM.

    Make the following modifications to the DAC161S997 EVM board:

    1. Swap C20 and C17. There is a mistake in the layout which switches these capacitors. This pretty much fully attenuates the HART signal before the gain stage so this is why you don't see any HART signal in the current output.

    2. Replace R11 with a 500 ohm resistor. The modem needs a minimum resistive load of 160 ohms.

    3. Remove C16. This causes attenuation of the HART signal. 1uF is a lot more capacitance than should be connected across loop for HART implementation.

    Here are the steps I took when connecting the EVMs together:

    1. Connect the power supply and load resistor to the DAC161S997 EVM.

    2. Set some current value for the loop using the DAC161S997 software, I used midscale.

    3. Connect GND of DAC874xH EVM to COM of DAC161S997 EVM and MODOUT to test point H9 .

    4. In DAC874xH software: Enable reference, enable BPF, and type some data to transmit then click 'generate read'.

    5. Measure the voltage across the load resistor to see the HART signal. Be careful to avoid ground loops. When I initially tried to measure with the GND of the oscilloscope connected to the low side of the sense resistor it created a ground loop with the computer I was using that was also plugged into AC power. To solve this issue I connected the scope ground to COM and measured across the resistor with two separate channels.

    Below is the result:

    Channel 1 is the positive side of the load resistor while Channel 2 is the negative side. M is (CH 1. - CH 2.) voltage across load resistor. The voltage is about 240mVpp across ~250ohm resistor or ~1mApp.