DAC161S997EVM: about sine wave amplitude of HART signal

Hello,

motoo nakayama said:

1. When HART modem output FSK amplitude at 700mVp-p to DAC161S997,
   the output of DAC161S997-9pin(OUT) drops to 70mVp-p
   and it monitors 300mVp-p at a point of the 250Ω Resistor.
   According to HART specification, it is written that FSK amplitude needs more than 400mVp-p.  
   Is it possible to gain FSK amplitude at 9pin(OUT) ?

The main HART requirement for a 2-wire transmitter is that the FSK signal is 1mApp on the 4-20mA current-loop. There isn't really any voltage implication where you have made your measurement as far as I know.

motoo nakayama said:

2. When DACout is 5mA, sine wave is output correctly.
   But when DACout is 4mA, it looks like one side of sine wave is not good.
   Do you have any solutions for outputting normal sine wave at 4mA ?

My best guess given the information we have so far is that your setup is consuming too much current for the loop current value to reach ~3.5mA when the 1mApp FSK signal is being output while the current loop is set to 4mA.

Dear Duke,

Thank you for your reply.

The sine wave problem was solved by removing MSP430 and ISO7421 on DAC161S997EVM that were consuming much current as you say.

About the FSK amplitude,

1mAp-p * 250Ω = 0.25Vp-p,

when the current sense resistor is 250Ω, FSK voltage swing must be 0.25Vp-p as the HART specification.

Is my understanding correct ?

motoo nakayama said:

About the FSK amplitude,

1mAp-p * 250Ω = 0.25Vp-p,

when the current sense resistor is 250Ω, FSK voltage swing must be 0.25Vp-p as the HART specification.

Is my understanding correct ?

Yes.