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DAC8760: 10V Output is not enough for PLC Analog Input

Cyrus Kelly
Prodigy 210 points
Part Number: DAC8760
Other Parts Discussed in Thread: DAC161P997, DAC161S997

This chip is supposed to be for 4-20mA process control applications.  However the specifications indicate the output voltage is not enough to work for a 4-20mA PLC analog input application. 

The 4-20mA instrument needs 10.5V minimum.

The PLC will have a 250R resistor to sense the output current of the instrument.  That generates a voltage drop of up to 5V (0.020A x 250R).

The wires to the instrument will also drop some voltage, although small.

The absolute minimum voltage output needed is 15.5V

The DAC8760 can only put out 10V.

Am I missing something?  Should I be looking at a different part for the PLC AI application?

Thanks for your help.

Cyrus

  • 0
    TI__Mastermind 46290 points

    Cyrus,

    I think there may be some misunderstanding here.

    The DAC8760 features both voltage and current outputs.

    The voltage output options are 0-5V, 0-10V, +/-5V, and +/-10V. Each of those output ranges have an optional 10% over-range, so in total there are 8 programmable 16-bit voltage output spans, provided at the VOUT pin. This is a voltage driver.

    The current output options are 0-20mA, 4-20mA, and 0-24mA. In this case, there is no over-range option, so you have 3 programmable 16-bit current output spans, provided at the IOUT pin. This is a current driver.

    VOUT and IOUT functionality are completely separate.

    When we consider the voltage developed at the point of load for a current output we look at the full-scale current value, the maximum load impedance (cabling included), and the compliance voltage headroom requirements of the device or circuit implementing the current source.

    So for your example with 20mA into a 250 ohm load, we will develop a potential of 5V, as you mentioned. DAC8760 specifies a compliance voltage of 2V, so if we want to drive 20mA into a 250 ohm load - DAC8760 needs at least 7V AVDD (this assumes no cable impedance). You can add whatever headroom you feel necessary for the cabling, but DAC8760 requires a minimum of 10V AVDD in order to function, so you could have another 150 ohms of cable impedance and still be fine with the minimum 10V AVDD.

    DAC8760 accepts up to 36V AVDD. So, for IOUT driving 20mA full-scale current, you could drive up to 1.7kOhms of impedance (load+cabling). If driving up to 24mA full-scale current, the maximum load impedance is ~1416.66kOhms. This is enough to drive all conventional PLC loads.

    Cyrus Kelly said:
    Am I missing something?  Should I be looking at a different part for the PLC AI application?

    The only concern I have left is that you said "AI" here. Typically, at least in TI, we use that for "Analog Input" module, so maybe I've missed your context somehow.

  • 0 in reply to Kevin Duke
    Prodigy 210 points

    Hello Kevin,

    What you are describing is what I would call a PLC Analog Output that drives a device (e.g. a valve) with a current of 4-20mA.

    What I am asking about is a PLC Analog Input.  The instrument (e.g. a pressure sensor) outputs a current of 4-20mA.  At the PLC, the current passes through a 250 ohm resistor and the voltage is measured by an ADC.  The instrument is usually powered by the loop.  It needs 10.5V minimum to operate, plus we need to allow for the voltage across the 250 ohm resistor (see attached picture).

    Cyrus

  • 0 in reply to Cyrus Kelly
    TI__Mastermind 46290 points

    Cyrus,

    DAC8760 is not an ADC, so it is not suitable for the AI module. I assume this is probably not your question.

    Sensor transmitters deploy both 2-wire, or loop-powered, and 3-wire or 4-wire architectures. The 3- / 4- wire applications basically still resemble the structures used in the PLC AO modules, but are typically only utilized by a sensor when the sensing element itself requires more quiescent current than the 4-20mA loop-powered architecture would allow.

    DAC8760 would service 3- / 4- wire PLC or sensor applications, however not be a fit for 2-wire sensor applications.

    For sensor applications you could consider DAC161S997 or DAC161P997. We also have some roadmap developments that you and I could discuss offline, as we have previously established email communications.