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DAC161S997EVM and HART signaling

Wayne Shumaker83987
Intellectual 330 points
Other Parts Discussed in Thread: DAC161S997EVM, DAC161S997

I'm attempting to interface a HART signal with the DAC161S997EVM and also need to modify how it connects. I noticed that the schematic in SNLU146.pdf is not complete and it appears the capacitors are not connected correctly. Namely C1 and C3 on the schematic are reversed compared to the DAC161S997 data sheet pinout.

I also have been simulating my desired circuit using the model in the dac161s997 data sheet, Figure 14. HART Signal Injection. If I simulate that circuit it appears to be correct. However if I test on the demo board, this does not seem to be totally accurate. My assumption, based on Figure 14, is that there is a virtual ground on the amplifier that mirrors the current to the output. The source current DAC is filtered with 3 15K resistors. It seems to me that the DC voltages on these nodes C1, C2, and C3 have a DC bias to them and don't match simulation or this model. Is there a more accurate model?

Is there anyway to get corrected schematic for the demo board, determine if the demo board is incorrectly wired, and if there is a better model for the HART filter stage. I have LTSpice circuit model I can make available. The simulation matches exactly what the data sheet shows in Figure 14. I just can't get the DEMO board to also match correctly.

In addition to the possible board errors, the capacitance added to the loop output attenuates HART signals and needed to be removed.

  • 0
    Intellectual 330 points

    Update: HART and Intrinsic Safe usage with this DAC.

    For anyone reading this thread. Unless the evaluation board, DAC161S997EVM, has been updated, it will not work with HART. It has a schematic error where pins C1 and C3 have been switched. The Caps on the board will have to be switched if HART is to be used. In addition, for HART, the large output Cap C16 and maybe C15 will have to be removed. Note that these Caps help the system stability, and their removal needs to be tested. In addition, for HART, the 200 Ohm resistor feeding the LDO and the LDO input Cap also affects the HART impedance and have to be fixed. I replaced the LDO with an alternative, non-TI part, that did not require large input capacitance.

    I ended up building my own DAC breadboard and have HART working with this DAC and passing HART physical layer testing. It is important that the analog supply be well isolated from the digital system to avoid noise coupling. For HART noise performance, this DAC is easier to smooth the current steps than other competing vendor DACs. This is because C1, the DAC filter Cap, can be increased independently from the HART input coupling.

    These capacitor and circuit changes were also required to have both HART working and to pass Intrinsic Safety requirements, where any capacitance on the Loop output has to be very small or zero.