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ADS1248: ADS1248 current in Power Down MODE

Noam Weidenfeld1
Intellectual 780 points

Part Number: ADS1248
Other Parts Discussed in Thread: ADS1218, SN74LVC2G157, PCA9535

Hi,

I configured the ADS1248 to power down mode by taking the START pin to low.

VDD is connected to 3.0V and AVD is floating (The analog power was shut down).

According to the datasheet I should expect a current les then 1uA over the VDD however the current is 200uA. (When I disconnect the 3.0V from VDD the current drop by 200uA)

Any idea why the current is that high?

Attached is the ADS1248 schematic.

Thanks

Noam

Analog Input iii.pdf

  • 0
    TI__Guru** 113765 points

    Hi Noam,

    It is not totally clear how you have the ADS1248 connected.  Is the 2.5V powered up and connected to the REFP0 input or the AIN0 analog input?  Is there a voltage at the AIN1 input?  If there is a voltage on these inputs and the AVDD is removed you are violating the absolute maximum input ratings.

    When you remove the AVDD power, the device will go into a reset condition.  You cannot use the 'power down' current in the datasheet for this condition.  What you are referencing in the datasheet is with AVDD and DVDD applied and the START pin low and that is considered power down.  Try applying AVDD (and DVDD) and then set the START pin low and measure the current.

    Best regards,

    Bob B

  • 0
    TI__Guru** 100775 points

    Noam,


    It is likely that the reference was programmed to be on during power-down. This would be approximately 180uA of current.

    In the Multiplexer Control Register 1 (MUX1). The VREFCON[1:0] bits control the internal reference. When the bits are set to 00, the internal reference is off. When the bits are set to 01, the internal reference is always on. However, when the bits are set to 10 or 11, the internal reference is on when a conversion is in progress and powers-down when the device receives a SLEEP command or the START pin is taken low. I believe that you have the bits set to 01, and you want them set to 10 or 11.

    When large capacitances are used for VREFOUT the reference may take a long period of time to turn on and settle to the final reference voltage. For that reason, there is an option to keep the reference on during power-down mode.


    Joseph Wu

  • 0 in reply to Bob Benjamin
    Intellectual 780 points

    Bob Hi,

    Actually the all board is powered down and there is nothing at the ADS1248 analog inputs.

    The VDD sources the CPU as well so I have to leave it ON.

    So all the power sources 5V, 2.5V and +/- 15V are OFF. Analog inputs are floating, only the VDD is ON.

    Noam

  • 0 in reply to Joseph Wu
    Intellectual 780 points

    Joseph Hi,

    Beside this register, is there any other registers that I need to take care of?

    Is there a sequence to power down mode or taking down the START signal is enough?

    Thanks

    Noam

  • 0 in reply to Noam Weidenfeld1
    TI__Guru** 100775 points
    Noam,


    First of all, sorry about marking the "TI thinks this is resolved" button. We've recently changed the system and there's no way for me to unclick it.

    Taking down the START pin low should be sufficient to put the device into power-down mode. There shouldn't be any other register settings or setup required for power-down mode.

    I've talked to Bob and we're still a bit unclear as to what you are doing. You are using node names that aren't in the schematic and pin names not in the device. I assume that VDD means DVDD, and is supplied by your 3V0 node, and that AVD is AVDD, supplied by your 5VA. I also assume that AGND and DGND are connected through the 0Ω resistor R527.

    For your test, you disconnect the analog supply from 5VA and you say the 3V0 supply drops by 200uA. Is that correct? Aside from the fact that disconnecting the supply isn't quite the same as powering down the part, I'm not sure where the extra current was going. If the device is powered down, the 3V0 current going into the device should have already been very low (less than 1uA). The best way to check the shutdown current would have been to check the current going into the device with an ammeter. Does the 3V0 supply power anything else? There is one label for the node going to an ADS1218_PDWN, but there's nothing else shown on the circuit.

    At this point, I think we'll need more information about the rest of the circuit and your test. There isn't anything that would obviously draw extra current into the device.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Intellectual 780 points

    Joseph Hi,

    All the analog power are down, 15VA, -15VA, 5VA and 2.5VA. Only 3V0 is ON.

    When I disconnect L9 (Connect the 3V0 to DVDD I measure a drop of 200uA.

    Meanwhile we noticed that the power down mode (Taking START pin to "0") works only if the pin Reset is high.

    BR

    Noam

  • 0 in reply to Noam Weidenfeld1
    TI__Guru** 100775 points

    Noam,


    I didn't get a chance to look at this today. I think I can use an ADS1248EVM and duplicate the test for current.

    However, I still am a bit unclear on your test. To be sure of your order, you:

    1. Power down all analog supplies. Do you set all analog supplies to 0V? or do you disconnect the analog supplies? Even with 0V on the analog supplies, there are connections between the analog and digital sections that might cause extra current to be pulled.

    2. Then you pull down the START pin to power-down the device. At this point what is the digital current? Did you check the digital current when the START pin is pulled down and the analog supplies are still on?

    3. Now you pull out L9 and check the 3V0 current? What was the before and after current? Is the difference exactly 200uA?

    Regardless, confirm this order and I'll try to test it the way you do.


    Joseph Wu

  • 0 in reply to Joseph Wu
    Intellectual 780 points

    Joseph Hi,

    Power down meas that all the DC-to-DC powers are disable except 3V0.

    The steps you mentioned are correct.

    1. I disabled all the analog powers.

    2. Pulled the START pin low (There is a chance that is was low before I puled it down).

    3. When I measured the total power on 3V0 it was 280uA,

    4. When I repeated all the above with L9 not assembled the total power was 80uA.

    As I mentioned in a previous mail, the Reset signal in power down mode was "0", When we pulled it high the current drop down. Is it make any sense?

    Thanks

    Noam

  • 0 in reply to Noam Weidenfeld1
    TI__Guru** 100775 points

    Noam,


    I set up the ADS1248EVM to test the current draw and I don't get the same numbers that you do.

    I started with measuring the AVDD and DVDD currents with ammeters. For the AVDD current, I measure the current going from leads 9 and 10 of J10. At the same time, I measure the current going to DVDD from leads 5 and 6, also from J10. I believe that AVDD goes to the supply of the ADS1248, and the pin is not used for any other function. For DVDD, there are currents feeding the ADS1248, but also the TPS3836DBV, SN74LVC2G157, and PCA9535. I'd note that I believe the first two devices draw almost no current, while the PCA9535 draws about 30uA with a 3.6V supply. I believe the ADS1248EVM runs DVDD at 3.3V

    I measure the device currents in several configurations:

    1. PGA=1, Data Rate=5, Reference off, ADC inputs shorted to Vbias: IAVDD=269.8uA IDVDD=203.9uA
        Power-down through SLEEP command: IAVDD=0uA IDVDD=32.8uA
        Power-down through START pin: IAVDD=0uA IDVDD=33.5uA

    2. PGA=1, Data Rate=5, Reference always on, ADC inputs shorted to Vbias: IAVDD=462.0uA IDVDD=203.9uA
        Power-down through SLEEP command: IAVDD=192.3uA IDVDD=32.8uA
        Power-down through START pin: IAVDD=192.2uA IDVDD=33.5uA

    3. PGA=1, Data Rate=5, Reference follows START, ADC inputs shorted to Vbias: IAVDD=462.4uA IDVDD=204.1uA
        Power-down through SLEEP command: IAVDD=0uA IDVDD=32.8uA
        Power-down through START pin: IAVDD=0uA IDVDD=33.5uA

    Note that the IVDD current is still a bit high. I believe that most of that current is supplied to the PCA9535. Also, there is a slightly different current supplied to the DVDD by pulling the START pin low. To pull the START pin low, I pull GPIO0 (pin 2) of J7 low. The difference in DVDD current is probably the pull down current from RA1A for START. Both AVDD and DVDD
    appear to shut down. In configuration 2, the reference voltage is enabled, adding almost 200uA of current.

    Then I change the configuration:

    4. PGA=128, Data Rate=2000, Reference off, ADC inputs shorted to Vbias: IAVDD=686.5uA IDVDD=256.3uA
        Power-down through SLEEP command: IAVDD=0uA IDVDD=32.8uA
        Power-down through START pin: IAVDD=0uA IDVDD=33.5uA

    5. PGA=128, Data Rate=2000, Reference always on, ADC inputs shorted to Vbias: IAVDD=878.1uA IDVDD=256.3uA
        Power-down through SLEEP command: IAVDD=192.5uA IDVDD=32.8uA
        Power-down through START pin: IAVDD=192.3uA IDVDD=33.5uA

    6. PGA=128, Data Rate=2000, Reference follows START, ADC inputs shorted to Vbias: IAVDD=878.1uA IDVDD=256.4uA
        Power-down through SLEEP command: IAVDD=0uA IDVDD=32.8uA
        Power-down through START pin: IAVDD=0uA IDVDD=33.5uA

    While the supply current was slightly higher, it was expected with the increase in data rate.

    After that, I completely disconnect the AVDD. The device still seems to respond to the the EVM interface, but there is no AVDD current.

    7. PGA=1, Data Rate=5, Reference off, ADC inputs shorted to Vbias, AVDD open: IDVDD=204.0uA
        Power-down through SLEEP command: IDVDD=32.8uA
        Power-down through START pin: IDVDD=33.5uA

    8. PGA=1, Data Rate=5, Reference always on, ADC inputs shorted to Vbias AVDD open: IDVDD=204.0uA
        Power-down through SLEEP command: IDVDD=32.8uA
        Power-down through START pin: IDVDD=33.5uA

    9. PGA=1, Data Rate=5, Reference follows START, ADC inputs shorted to Vbias AVDD open: IDVDD=204.0uA
        Power-down through SLEEP command: IAVDD=0uA IDVDD=32.8uA
        Power-down through START pin: IDVDD=33.5uA

    I also change the PGA and data rate to check currents:

    10. PGA=128, Data Rate=2000, Reference off, ADC inputs shorted to Vbias, AVDD open: IDVDD=204.0uA
        Power-down through SLEEP command: IDVDD=32.8uA
        Power-down through START pin: IDVDD=33.5uA

    11. PGA=128, Data Rate=2000, Reference always on, ADC inputs shorted to Vbias AVDD open: IDVDD=204.0uA
        Power-down through SLEEP command: IDVDD=32.8uA
        Power-down through START pin: IDVDD=33.5uA

    12. PGA=128, Data Rate=2000, Reference follows START, ADC inputs shorted to Vbias AVDD open: IDVDD=204.0uA
        Power-down through SLEEP command: IAVDD=0uA IDVDD=32.8uA
        Power-down through START pin: IDVDD=33.5uA

    Through each of the tests, I don't see anything unusual. I didn't check to see what happens when AVDD is connected to ground. Regardless, if you short the AVDD line to ground, you might get something marginally different. I'd note there are some digital circuits within the device. This could draw extra current when the device has AVDD=0V. Imagine you have a digital line going into the ADC analog section, this still has to pass from DVDD to AVDD with some level shifters. This is important when AVDD is not the same as DVDD.

    Regardless, it looks like the tests that we've run here show that the device power-down appears to work normally. Let me know if you have any questions. If I've missed something in my test, let me know.


    Joseph Wu

  • 0 in reply to Joseph Wu
    Intellectual 780 points

    Joseph Hi,

    Thank you for your detailed information.

    We succeeded to bring the power down in our product so it been a good practice.

    BR

    Noam