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DAC161P997 abrupt transition from low to high loop current causes invalid output

Karl M
Prodigy 200 points
Other Parts Discussed in Thread: DAC161P997

Hi all,

We have a design which uses the DAC161P997 to generate a 4-20 mA current loop output. It has been discovered that invalid output results under a specific set of conditions, and I'm wondering if this is to be expected or if it can be corrected.

The implementation is similar to the loop-powered transmitter application in the datasheet. The DAC circuit is optoisolated from the host processor. With loop supply present and the host MCU powered off, the DAC generates the expected error output (3.375 mA). When the MCU boots, it sets the output to 4 mA. With a bench power supply and a bench ammeter forming the loop, everything works perfectly anywhere within a a range of about 7 to 36 volts loop voltage, and over 4 to 20 mA.

It goes wrong  when a 500 ohm resistor is inserted in the loop (to emulate the customer's sense resistor), and then only when the system tries to jump immediately to a reading near full scale (20 mA) from the initialization value of 4 mA. I have replicated this with my diagnostics for the driver board, where three keys set presets of 4, 12, and 20 mA respectively. If I power up the board and step from 4 to 12 to 20, everything is great. If I step from 4 directly to 20, it drops to a slightly varying output around 3.5 mA. If I then select 12, the output becomes 12 mA, and I can subsequently step to 20 mA with no problem.

So, the DAC is not locking up, as it can always be made to recover by lowering the current setpoint, and then raising it again, but not too abruptly. I can work around this in the driver firmware easily enough, by turning any change into a series of smaller changes. I was just wondering if this is expected behavior for this part.

Thanks,

---Karl

  • 0
    TI__Genius 14270 points

    Hello Karl.

    I expect that the 3.5mA is the error condition current which occurs when the system cannot maintain the loop. Does the ERRB pin indicate an error?  With 20mA across 500ohms, you will have a drop of 10V. What was the loop voltage when you were evaluating the behavior? Also, is where is the 500ohm sense resistor - is it on the high side of the loop before the transistor or is the resistor on the low side after the OUT pin?

    Regards,

    Chris O

  • 0 in reply to Chris Oberhauser66
    Prodigy 200 points

    Hello Chris,

    The varying ~3.5 mA output is definitely not the same as the stable 3.375 mA which I see when the DAC is not receiving any data. It appears to be the average of the DAC switching between the error current and a higher unstable value. This design does not have ERRB brought out, so it was a bit tricky to probe, but I did so and confirmed that it is pulsing in the broken condition.

    The loop supply in the customer's installation was 24 volts. I replicated the problem anywhere from the minimum at which the DAC would operate up to 36 volts with no change. Our circuit functions down to roughly 7 volts, so a 24 volt loop supply is about ideal with a 500 ohm sense resistor.

    The circuit containing the DAC is totally isolated from everything else, so it makes no difference where the resistor is located in the loop. It's the same series circuit either way.

    We appear to have found an explanation and solution. It turns out that we used more capacitance on the input to the LDO powering the DAC from the high side of the loop than the example in the datasheet shows (47 uF electrolytic). Apparently the DAC is sensitive to this, even through a 100 ohm isolation resistor. Changing the cap to a 1 uF ceramic seems to have eliminated the problem.

    We have some extra protective components in the circuit, but we removed or bypassed most of them in the process of narrowing this down with no effect. These include a series polarity protection diode, series PTC fuses, series ferrite beads, low value high voltage bypass caps, and a 36 volt TVS. We're also using a pretty beefy pass transistor (FZT692BTA), as we were burned (pun not intended) by the power dissipation in a SOT-23 at 20 mA and 36 volts. This part does have significantly more gain than the 2N3904 mentioned in the datasheet; perhaps that has something to do with this issue.

    I should also mention that the power for the DAC coming from the LDO is totally clean, even in the broken state. The high side of the loop itself shows definite repetitive spikes during the failure, while the input to the LDO has some variation on it (but less amplitude and more smoothed out by the isolation resistor and cap, as expected).

    Another thought we had was the addition of an isolation diode between the loop and the LDO input filter cap. This would prevent the cap from discharging back into the loop, which appears that it might be what is happening. Any thoughts?

    Thanks,

    ---Karl

  • 0 in reply to Karl M
    TI__Genius 14270 points

    Hi Karl.

    The ESR of an electrolytic cap could affect the LDO; if that is the case than a diode would probably not help.

    Regards,

    Chris O