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Nano Ampere Current Sensing Hardware Required??

Umer Daraz
Prodigy 200 points

Other Parts Discussed in Thread: ADS1281, ADS1271

Hi All,

I want to discuss my application requirement with TI application experts. I have a sensor that output is in current form. The current is in the range of +/- 24 mA maximum and signal rate of change is 1.0 KHz. The environment temperature is 60C to 70C. My required precision is maximum +/- 10 nA. The sensor will be at a 10 inch of distance from electronics circuit. Output of selected ADC should be in SPI. I am interested in TI products overall. Kindly help me in hardware design.

Thanks alot

Umer

  • 0
    TI__Mastermind 23050 points
    Hi Umer,

    This is a challenging design goal, but based on your requirements you are going to want to look at 24-bit or greater delta sigma converters. If we assume a standard +/-2.5V input range (5V full scale), then you need to be able to resolve down to about 1uV. Whatever amplifier you use is going to have greater offset than this so calibration will be absolutely necessary. Even when using a 24-bit converter, you're not likely to get true 24-bit resolution because some will be lost to noise and settling error. You're going to need a transimpedance stage on the front end to convert the current signal to a voltage, and you will want to use a high speed amplifier to get decent settling into the converter. The op-amp selection should come after you choose an ADC, but once you have the ADC we can provide support regarding the design and selection of the op-amp.

    I'm not sure what kind of sample rate you're looking for, but if 4ksps is sufficient then you might be interested in the ADS1281, which is a 31-bit converter with very good SNR (118dB at 4ksps). On the 24-bit side you might want to look at something like the ADS1271, which has a max sample rate of 105ksps. You could potentially over-sample your signal and do some averaging to further reduce the noise contribution if this is going to be operated in a high noise environment. Both of these devices are SPI compatible. I must give a disclaimer, the delta-sigma portfolio is not exactly my area of expertise, so if you'd like me to redirect you to their forum they may be able to recommend something better suited to your system. Once you find the converter that fits best for you you will want to get in touch with the high speed amplifiers group. They will be able to recommend a suitable op-amp and help you with the design of the transimpedance amplifier.
  • 0 in reply to Zak Kaye
    Prodigy 200 points

    Hi,

    Some other requirements i want to share with you. My sensor output signal rate is 1KHZ, so i think ADS1271 is more suitable due to 105ksps. Kindly find attachment for design and layout limitations but your suggestions are welcome.

    Regards

    Umer

  • 0 in reply to Umer Daraz
    Prodigy 200 points

    Hello,

    I am waiting for your reply, kindly required your help regarding circuit design. If my question is not relevant to your expertise then kindly forward to any other expert in this field.

    Thanks and Regards

    Umer

  • 0 in reply to Umer Daraz
    TI__Mastermind 23050 points
    Hi Umer,

    What does the sensor you are using look like? What excitation voltage does this sensor require? Is this a two-wire sensor with the ability to ground one node? There are a few different ways to approach this depending on the sensor.
  • 0 in reply to Zak Kaye
    Prodigy 200 points

    Hi Zak,

    Sensor type is accelerator, excitation voltage is +/- 15V, output is 2-wire and one node is short with the +/- 15V GND signal.

    Waiting for your help in circuit design.

    Thanks

    Umer

  • 0 in reply to Umer Daraz
    TI__Mastermind 23050 points

    Hi Umer,

    There are a lot of different  things to consider here , but I think I have some information to help you get started. To start, you need to convert your current signal into a voltage with as high fidelity as possible. You could take a couple different approaches:

    1) You could use a transimpedance amplifier or an op-amp configured as a transimpedance amplifier. Here is a TI design for a photodiode transimpedance amplifier that covers the basics of how to design this section:  

    2) You could use traditional current sense methods using an instrumentation amplifier. If you are unfamiliar with current sensing, then this series provides a good foundation: www.eetimes.com/document.asp

    Once your sensor signal is converted into a voltage, you will likely still need to condition the signal. Additionally, since you need a high resolution delta-sigma converter, you are going to have to convert the singled-ended signal into a differential voltage. The simplest way to do this is to use a Fully Differential Amplifier (FDA).  If you are unfamiliar with FDAs, then we have some TI precision labs trainings that explain their functionality and how to use them, as well as considerations for driving converters. You can find these presentations here: 

    If you'd like to know more about driving ADCs and how to design for the best accuracy, then these presentations walk through some of the techniques we have developed to optimize the input amplifier. Although these presentations discuss SAR converters, the modeling principles are the same for delta-sigma and the tools still provide a useful starting point. If you were to use a converter with an internal PGA or input buffer, then this would greatly simplify the front end amplifier design.

    Summit - 2017 Part 1 - Amplifier Front End- 5-16-2017.pptx

    Summit - 2017 Part 2 - Driving the ADC - 6-7-2017.pptx

    Once you have a preliminary design, feel free to post it here and I can take a look and offer some feedback and help with some of the details.

  • 0 in reply to Zak Kaye
    Prodigy 200 points

    Hi Zak Kaye,

    Thanks for your great effort, I study all stuff and complete my design and then will send you for review,

    Regards

    Umer

  • 0 in reply to Zak Kaye
    Prodigy 200 points

    Hi Zak,

    First of all allow me to thankyou for your help. The material you provided was a great help in developing the basic understanding of the subject material.
    So thankyou!!!.

    However, i have run into yet another problem. The limited amount of data provided by our customer for the sensor has presented yet another design constraint which seems beyond my technical knowledge and requires your expert opinion. The output current parameters for the sensor are as follows:


    Output Current : +/- 10nA to +/- 100mA.
    Supply Voltage : +/- 15V.


    Our design goals then become:

    PARAMETER : GOAL
    ------------------------------------------------------
    Vomin (±10nA)                                             : ±100mV
    Vomax (±100mA)                                         : ±4.9V
    Sensor Maximum Current (ISMax)              : ±100mA
    Sensor Minimum Current (ISMin)                : ±10nA
    Sensor Supply Voltage                                : ±15V/500mA


    Can transimpedance amplifiers be utilized for the above mentioned values. Currently I am stuck at this range as i cannot find any solutions that can cater this current limit (+/-10nA to +/-100mA). Also the input voltage for the amplifier circuitry is also a constraint as only +/-15V is available.


    Your expert opinion in this regard, any help in product suggestion and design will be greatly appreciated.

    Best Regards,
    Umer

  • 0 in reply to Umer Daraz
    TI__Mastermind 23050 points
    Hi Umer,

    I am sorry but I do not have a solution to offer you. Please understand that what you are asking for is a professional level of design consultation, and this is well outside the scope of what this forum is intended for. We are here to help with your application issues for operational amplifiers, but it sounds to me like you are looking for a full system solution and you may want to consider contracting a professional consulting organization. Achieving 10nA resolution on a 100mA signal is a very difficult task, especially once you start considering all the noise sources in the system. This kind of performance is characteristic of precision bench-top meters, and the intimate details behind the design of these is far from our area of expertise.

    One thing I would like to make clear is that being able to measure down to 10nA accurately and having 10nA resolution on a 100mA signal are very different design goals. If for example you only needed 100uA resolution when in the 100mA range then this becomes a much more practical design and I'd be happy to offer you some suggestions on how to achieve this. Otherwise, I am sorry but this design is outside the scope of what I can support.
  • 0 in reply to Zak Kaye
    Prodigy 200 points

    Hi Zak,

    In my previous mail I provided you with some of my design constraints but after reading your reply I realized that it might have sounded like I was looking for a design solution from you which is not the case at all. 

    My purpose in consulting you and providing you with those design constraints was because I was interested in your recommendation of a TI product (such as a trans-impedance amplifier, fully differential amplifier, fully differential trans-impedance amplifier or current sense amplifier) that can handle those design constraints (Amplifier Input Current: +/-10nA to +/-100mA, Supply Voltage: +/-15V).

    I realize that designing with these constraints is a very difficult task and that is why I am seeking the advice of professionals as a starting point for my design. I apologize if my previous mail was unclear on the matter, however, I do hope you can 
    provide a recommendation for a TI product that will be a right starting point for my design.

    Best Regards
    Umer

  • 0 in reply to Umer Daraz
    TI__Mastermind 23050 points
    Hi Umer,

    I think you need to think about this from a system level first to decide on an approach that is feasible before you start looking at specific amplifiers. You don't usually start with the amplifier at the front of the signal chain, but rather with a converter that satisfies all of your criteria (SNR, full-scale range, resolution, etc.). From there you would select an amplifier and a configuration that can properly drive said converter, and then you would start looking at how to interface with the sensor because at that point you would have a more bounded system with defined input/output ranges.

    Knowing the signal range and the supplies is a good start, but there are a lot of other things to consider for a design like this. For example, let's assume you decide to use a 24-bit converter that can handle a +/-10V input range (the wider the range you have, the better you can resolve lower signal levels, though there is a catch: wider range equals more noise). That gives you 20V of full scale range, which means your LSB size is 20/(2^24) = 1.2uV. If 100mA translates to 10V, then assuming linear scaling 10nA would translate to 1uV. At first glance it appears you can almost get 10nA resolution, but then you have to consider noise. Let's assume you have a converter with an SNR of 110dB (which is quite good!). With a full scale range of 20V, this puts the noise floor of the converter at 22uVrms, well above your 1uV signal, and this isn't even considering the noise that your signal chain is going to add, which is very likely going to be greater than 1uVrms.

    Frankly, it is difficult for me to recommend something to you because I'm not sure how to come anywhere close to your target, even if I could give you an ideal op-amp. To put this into perspective the Keithley 7510, a $5000 instrument, is only just able to achieve 10nA resolution on a 100mA signal with a maximum refresh rate of 26Hz...