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log114: Using precision current source from CC2640r2 with LOG114

PATRICK OHARA
Genius 3205 points
Part Number: LOG114
Other Parts Discussed in Thread: ADS8866, OPA350

Hello,

I'd like to use the precision current source on the CC2640r2 as the reference current input to the LOG114 (I1 pin 4). This allows me to adjust the current source in firmware from 1 uA to 10 mA. This give me some "scalability" for my photodiode input. Do I need to condition this signal in anyway? Or can I just connect the precision current source from the CC2640r2 directly to pin 4 on LOG114 - no resistors, capacitors, buffer amps, etc.?

Thanks,

Patrick

  • 0
    TI__Guru* 93105 points

    Hello Patrick,

    The CC2640r2 datasheet provides minimal information about the programmable current source specifications. It lists in Section 5.23 a current range of 0.25 to 20 uA, with a resolution of 0.25 uA (VDDS = 3.0 V). I am not sure how you would go about adjusting the current beyond the 20 uA specified maximum. There isn't any information about stability, noise or what is required from a circuit standpoint when using this current source function. It appears to be more of a general purpose current source versus a precision current source. The LOG114 is a precision log amplifier and the precision performance will definitely be affected by the quality of the reference current source.

    It appears that the CC2640r2 programmable current source could simply be connected directly to the LOG114 I1 or I2 input, without any additional circuitry. The addition of a capacitor connected from the CC2640r2 current source pin to ground might help with noise on the pin, but then there is the issue of maintaining stability.

    The CC2640r2 is supported on TI's Bluetooth Low Energy E2E forum. They may be able to provide more information about the CC2640r2 programmable current source electrical electrical characteristics and how to best apply the function for best performance.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    Genius 3205 points

    Hi Thomas,

    Thank you for your reply - very helpful. In the end I decided, for a variety of reasons, not to use the CC2640r2 precision current reference as the input to the LOG114. My final implementation is shown below. I'd be interested in comments.

    Thanks again,

    Patrick

  • 0 in reply to PATRICK OHARA
    TI__Guru* 93105 points

    Hello Patrick,

    Nice LOG114 schematic! Just about everything looks good to me. There are some minor points that you may need to consider:

    • C30, a 10 uF capacitor, is connected from the 2.5 V reference pin 16 to ground. The LOG114 datasheet doesn't show a capacitor connected at this pin in any of the application circuits. The concern is the stability of the internal reference amplifier with a large capacitive load connected to its output. It may be okay, but I don't have any information indicating it will be. I see no issue with providing a place for C30 and if the reference circuit remains stable with it, then it would be useful for noise filtering. Note there is also C31 down the line but it is isolated from the reference output by R15 so it likely isn't an issue.
    • There will be some optimal values for R16 and C32, and they will depend on the ADS8866 operating setup and the LOG114 VO4 op amp characteristics. You may find that some optimization of the R and C values will be required to achieve the highest performance from the converter.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    Genius 3205 points

    Hi Thomas,

    Thanks for the great comments! I wondered about the 10 uF capacitor at C30 also. I added it as it is suggested by Bonnie Baker in the snippet of text from her TI technical article, "Best of Bakers Best - SAR ADCs" as shown below. I did not investigate the OPA350 CMOS OpAmp that she refers to as it may have better ability to drive a capacitive load. In the context of the circuit that I show, I'll take your advice and remove C30.

    As for optimizing R16 & C32, I'll make these larger package sizes to accommodate easy changing for optimizing.

    Thanks again,

    Patrick