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ADS1248 - SBAA211 - Universal Input Capability - Analog Input Connections (0-10V)

Rafael Guayer
Prodigy 20 points
Other Parts Discussed in Thread: ADS1248

Hi,

First I would like to thank you for the great Application Report SBAA211. I am designing a very similar system and the document has been really helpful. I am using ADS1248 for reading RTD, thermocouples, current source (0-20mA) and voltage sources (0-10V) from the same connector.

I'm having some trouble understanding the Analog Input Connection (0-10V) on Application Report SBAA211 - Item 3.6, Figure 9, page 9.

To respect the common-mode input range, 0.1mA from AN1+IDACn will result on 200mV bias on pin 3 of the connector (200mV drop on R3) and 0.1mA from AN2+IDACn will result on 200mV bias on pin 2 of the connector (200mV drop on R2).

With the voltage source connected to pin2 (Vsource+) and pin 3 (Vsource-), if Vsource- is 0V and the ground is not isolated/floating (Vsource- = AVSS), it would connect the 200mV bias with the 0V.

What would happen then? In my understanding the IDACn connected to AN1 will keep the 0.1mA sourcing but the voltage would drop to 0V, not respecting the common-mode input range of AVSS + 0.1V + (Vin)(Gain)/2 (ADS1248 datasheet Page 3).

On pin2 there is a 18k resistor that separate Vsource+ from the 200mV bias, so here I think the voltage would add to the 200mV bias, and for a 10V input I would see a 1.2V after R1 (there is a voltage divisor (2k/(18k+2k))=0.1). Is that assumption right?

I would also like to point some differences in the schematic figures and the switch position table. In Figure 10, SW3 and SW2 are closed, but Table 1 says that they should be open. If SW3 is closed, R3 will be in parallel with R18 with Req=95Ohm, and a 1mA current from AN1 would make 95mV, lower than 100mV common-mode input range. With SW2 closed, depending on the 4-20mA source voltage, there will be some current leakage trough R2.

Would you please clarify that?

And what about circuit protection, would you please recommend a solution? I am only placing zener diodes (SMBJ10CA and SMBJ5.0A).

Thanks in advance.

  • 0
    TI__Guru* 97415 points
    Rafael,


    Thanks for your post. I didn't write the note, but I think I can explain it. If you have further questions, you can post again and ask again.

    The measurement input to J0 is meant to be single ended, so the only input is pin2 from J0 measuring the top of R2 with AIN0. The negative side of the input is measured by the IDAC going to R3, with pin 3 of J0 open. This would always be at 200mV above the ground node.

    With this setup, it has the effect of giving a gain of 1/10 V/V with an offset of -20mV.

    At 0V input, the top of the R1 acts as a virtual ground, so the IDAC current of 0.1mA reacts to 18k||2k. The ADC would measure AIN0=0.18V and AIN1=0.20V. This gives a measurement of --0.020V.

    At 10V in, the voltage at AIN0 is calculated to be 1.18V, with AIN1=0.20V as before. This gives a measurement of 0.980V.

    As for Figure 10, I believe you are correct, the circuit of Figure 10 doesn't quite work. However, I think you only need R2 to be open. If I read this correctly, R18 is only going to be used for lifting the input common mode. In that case, R3 doesn't contribute much to the circuit either way. Still, I think you would want S2 and S3 open for the measurement.

    When it comes to the input protection, I don't have many good suggestions. Often, series resistance helps prevent damage from ESD strikes by limiting the current that the device can see. For large protection diodes, the leakage current is often the concern. It depends on the measurement, of course. With a 4-20mA detector, a leakage current becomes an offset error. In a 0-10V input measurement, you don't have a problem as long as the output impedance of whatever you are measuring is low.

    If you have a particular specification you are trying to meet for circuit protection, post what you are looking to meet as well as what you are measuring and maybe we can come up with some solutions.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 20 points
    Hi Joseph,

    Now I understand that the current and voltage connections are for single ended readings and that there is the need for another connection to GND.

    I solved the problem using one independent circuit for each type of input (one for RTD, one for thermocouple and one for current and voltage) and multiplexing the connectors with analogic CMOS mux/demux with low Ron and some mosfets.

    This way I could implement filters and gain for each independent circuit getting better resolution and noise immunity.

    Thanks for the support.