DC979A 5V reference from DC590B

Hello 

I am looking to work with the following DC979A (eval board for LTC2442) 

I have a question regarding powering the 5V, LT1236 reference of the DC979A from the DC590 USB serial controller. 

Page 3 of the DC979A demo manual mentions the following 

"VCC: This is the supply for the ADC. Do not draw any power from this point. External power may be applied to this point after disabling the switching supply on the DC590. If the DC590 serial controller is being used, the voltage must be regulated 5V only, as the isolation circuitry will also be powered from this supply. See the DC590 Quick Start guide for details."

1. I looked at the DC590B schematic. There is no switching supply on the DC590. Its seems like they are all linear regulators. If I use the DC590, the are only options are 2.7V, 3.3V and 5V for the VCCA which powers the VCC rail of the DC979A. Here in lies my confusion. If the max supply when powered from DC590B is 5V and this rail powers the LT1236, how can the LT1236 provide 5V for the reference. This means the LT1236 should have zero dropout. Am I missing something in my understanding here? 

2. Also since the LTC2442 can only have a VCC range between 4.5V to 5.5V, I am confused as to how I use the DC979A being powered from the DC590B? Is the DC590B only for communication and the supplies all have to be external? 

Thanks

Raghu

  • Just wanted to add that the LT1236 is powered from VIN rail and not VCC rail. But VIN is is also 5V coming out of the isolated  LT3439 transformer driver. So VIN = 5V and not enough  headroom for LT1236 to operate. So my misunderstanding of point 1 above still remains. 

  • 0
    •  Analog Employees 
    on Oct 15, 2021 12:57 AM in reply to raggers

    HI Raggers,

    This is the board of the DC590B:

    14-conductor ribbon cable is connected from DC590B using J4 of this board.

    This is the J4 of DC590B

    which will be connected to J1 of the DC979A.

    Looking at this, VIN line is connected to V+ pin of J1 of DC979A which is connected to V+ pin of J4 of DC590B.

    V+ pin of J4 of DC590B is connected to V+ line in DC590B board which we should put external supply (10V to 12V) as what indicated in the DC590B board image.

    The VIN line of the DC979A should have enough voltage to power the LT1236 with this setup.

    Regards,

    Mon

  • Hi Mon

    Thank you for your quick reply. Yes. That makes sense. I would like to ask two follow questions 

    1. To work with the DC979A/LTC2442 board with 5V reference, an external supply (7V-12V) has to be used. USB cannot power the entire setup. Is this a correct statement? 

    2. Our end application has 5V as the maximum supply rail. If this is the case, I can replace the LT1236 5V reference with a 4.096V or 3.3V reference like the ADR4540 or ADR4533 reference. The minimum noise floor of the LTC2442 is 220nV rms. So this would still give me 

    log (4.096/220e-9)/log(2) = 24.15 bits or

    log (3.3/220e-9)/log(2) = 23.8 bits  

    of resolution. Is my understanding correct? 

    Thanks

    Raghu

  • Please ignore my last post. I do not think I read your reply well and followed up with two questions that were obviously not well thought off. Apologies!