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Original question:

PGA281: Below 10nA current measurement range?

PGA281: Is there any replacement for PGA281

Ahmet Inan
Intellectual 440 points
Part Number: PGA281
Other Parts Discussed in Thread: PGA280, OPA192, ADS1252, INA851, PGA112, PGA113, OPA2192, ADS1220, INA821, DAC1220, INA818, LMH6321, ADS1115, OPA350

Hi TI Technical Support Team,

My aim is to measure bidirectional 100nA - 100mA current range. The voltage range is ±5V. I read the TI reference design that explains 10uA - 100mA current measurement with PGA281 and read the post in this forum regarding 10nA measurement with PGA281 (where suggested answer was to cover 10uA - 100mA with the circuit given in reference design and covering 10nA - 10uA with another circuit). 

When I searched for PGA281 in Digikey and TI, it seems to be out of stock and lead time is longer. Could you please suggest me some other way/IC to measure the 100nA - 100mA range? If there is not a replacement for PGA281, do you think using 6 reed relays to switch gains (1M, 100k, 10k, 1k, 100, 10) could work to measure this 6 decade current range?

Thank you for your help,

Ahmet

  • 0
    TI__Guru 52106 points

    HI Ahmet,

    Unfortunately there is no pin-to-pin replacement for the PGA281 (or the PGA280).  Currently, the PGA280/PGA281 is the only precision programmable gain amplifier family with fully-differential output.

    What is the maximum input common-mode voltage in the application?  What are the voltage supply requirements?  If you were to build the circuit discretely, what is the part number for the reed relays you intend to use? Do you require a fully-differential output or is single-ended output acceptable? 

    Thank you and Regards,

    Luis

  • 0 in reply to Luis Chioye
    Intellectual 440 points

    Hi Luis,

    Thank you for your answer. I have attached the schematic for measurement part I am thinking to use. It is my initial design. SW1-6 are reed relays. I will use ±12V voltage supplies for the circuit. The output will be single ended. Opamp input voltages will always be between 0.1V - 1V range (U1A) and 2.4V - 2.6V range  (U1B). I will control the reIays based on ADC reading. I have attached datasheet for the reed relay I am planning to use.

    Thank you for your help,

    Regards,

    Ahmet

    eu.mouser.com/.../3205100012e-1633484.pdf

  • 0 in reply to Ahmet Inan
    TI__Guru 52106 points

    Hi Ahmet,

    This approach could work, since the contact resistance of the reed relay is low at the 150mOhm range.

    What is the maximum voltage compliance of the current source circuit?  I only ask to ensure that the voltage at the input of the OPAx192 buffer does not inadvertently exceed the voltage supplies of the OPAx192.  For example, if inadvertently, a 20mA current flows through the1kΩ Rsense resistor prior the user selecting the correct 100Ω resistor.  

    The input current of the OPAx192 is typically ±5pA and ±20pA max at 25C.  Is the OPA192 PCB board going to be subjected to a wide temperature range? The input bias current is ±5nA (maximum) at hot temperature, where the current starts to be high above +75C.  The bias current over temperature is shown on Figure 19.  If the input bias current over temperature is a concern, let me know:

    What is the voltage of the reference you intend to use for the ADS1252?  Let us know if you require assistance selecting the RC filter to drive the ADC or would like other suggestions.

    Since the ADC is powered with a +5V supply, the second stage inverting amplifier could be replaced with a +5V amplifier to avoid risk of overdriving the ADC.  Alternatively, a fully differential amplifier (FDA) to make use of the full range of the differential ADC.

    There are other possibilities using FET input instrumentation amplifiers, such as the INA851, with overvoltage protection; or the PGA112 or PGA113 in a combination a high impedance input buffer driving a single-ended ADC.

    Thank you and Regards,

    Luis

     

  • 0 in reply to Luis Chioye
    Intellectual 440 points

    Hi Luis,

    Thank you for further helping me. 

    The current source circuit is also supplied by -12 and + 12V. It will not exceed the input voltage specs of OPA2192.

    The PCB will always operate in cool enviroment. The temperature cannot rise above 25-30C. 

    Actually, I was going to use ADS1252, but then looking to Digikey stocks (ADS1252 is also out of stock) I decided to use ADS1220. The AVDD and DVDD of ADS1220 will be supplied seperately with 5V. I will use LM7805s for this purpose. (the inputs to LM7805 will be 12V from circuit)

    I added my initial design with current source part (without power supply and ADC part). My overall aim is as follows; I am trying to analyze battery with constant voltage and constant current tests connected to sensor socket on the PCB. The voltage that can be applied to battery is between -5 and +5V. I am generating it using DAC1220 and U4A.During constant voltage (CV) test, I am measuring the current following through battery (charge or discharge currents) using reed relays SW1-SW6. During CV, I am measuring battery voltage using three opamp INA and supplying it back to U1B. I can use INA818 or INA821 instead of three opamp design also. I am boosting the opamp output current using LMH6321 and using CL pin, I set the current limit approximately to 100mA. Also, battery voltage are mapped to ADC input range using U5B. I am planning to add clamp diodes to protect ADC inputs. During Constant current test, the desired current is also set by DAC1220 and U4A. This time the feedback to U1B is from output of U6B.  The schematic does not include bypass capacitors, I will put them in my final design.

    I have following questions regarding my design.

    1. Do you think this methodology could work? Do I need to modify my feedback loops to U1B to improve stability?

    2. I have no experience in designing RC filter for ADC input. I previously used TI 16 bit ADS1115 ADC but just added clamp diodes for overvoltage protection and 100 ohm resistor between input of ADC and measurement point. How do I have to design RC filter for my circuit?

    3. For DAC1220, I am following DAC1220EVM board schematic, it refers to using REF1004-2.5 for Vref. I am planning to use MCP1501 - 2.5V as I already have it. Do you think it can badly affect the accuracy of DAC1220? Should I move to REF1004?

    Thank you for helping me.

    Regards, 

    Ahmet

    MCP1501-2.5V datasheet: ww1.microchip.com/.../20005474E.pdf

  • 0 in reply to Ahmet Inan
    Intellectual 440 points

    demodesign.pdf

    I am also adding pdf of my schematic, as there is chance the picture is low resolution.demodesign.pdf

  • 0 in reply to Ahmet Inan
    TI__Guru 52106 points

    Hi Ahmet,

    I will review in detail and provide suggestions.  I will get back to you by late this evening or by end of day tomorrow at the latest.

    Thank you and Regards,

    Luis

  • +1 in reply to Luis Chioye
    TI__Guru 52106 points

    Hi Ahmet,

    Thank you for the additional information and schematic for the gain amplifier stage, below a couple of general comments, although the queries are more related to a battery tester:

    • Regarding the stability of the loop:  I have not performed a detailed stability analysis, but may need to ensure the loop feedback is negative.  You may also need to perform a detailed open-loop small signal analysis to ensure there is enough phase margin to guarantee stability. Looking at the feedback of the loop, it appears that the feedback loop may have a double inversion, producing positive feedback. It appears the loop starts at the inverting terminal of U1B (OPA2192), the input of the current buffer U3 (positive), the battery, the non-inverting terminal of U6B buffer, and the inverting terminal of U7A).

    •  The ADS1252 is a Delta-Sigma converter. The application note SBAA086 discusses how to model the ADS1252 inputs.  The effective input impedance is a function of the Delta-Sigma modulator sampling frequency and tends to be relatively high, at the 460kΩ to 96-MΩ range, so the inputs tend to be easy to drive, and there is some flexibility on the RC Filter.    As long as the impedance of the R-C-R filter is not to high to interact with the input impedance of the ADC, causing significant attenuation, and as long as the the driver op-amp is stable driving the capacitive load, there should be no issues.  In general, using a 1kΩ - 1nF - 1kΩ may be sufficient, but you could adjust the R-C-R filter corner frequency depending on the required bandwidth.  Alternatively, you could consider also the ADS1220 delta-sigma, which incorporates an internal PGA (programmable gain amplifier) and relatively high input impedance allowing plenty of flexibility selecting the RC filter. There is no need to worry about driving the ADC sample-and-hold, since the ADC incorporates a front-end.  The ADS1220IPW is in stock.

    • Instead of using diodes or clamps to protect the +5V inputs of the ADC, consider replacing the difference amplifiers op-amps U7A and U8A with +5V operational amplifiers powered with the same supply of the ADC. This will ensure the output of the op-amps are limited to +5V on any fault condition. Since op-amps U7A/U8A have a 10kOhm resistor at the inverting terminal, the current to the op-amp input is limited through the 10k resistor and no damage will occur to the +5V op-amps when the previous output stage is at the +12V rail supply.  I agree on possibly replacing the 3-op-amp INA with an instrumentation amplifier to simplify your design; eliminating some of the precision resistors and increasing accuracy of the system.

     

    • On the DAC1220EVM, the REF1004 is a shunt/Zener reference buffered with the OPA350 on the DAC1220EVM.   It may be best you submit a new query to the experts on the data converter forum regarding the DAC1220 reference requirements and recommendations.  There is some discussion regarding sensitivity on the DAC1220 reference and bypass capacitor on the DAC1220 datasheet under the Layout Recommendations section on page 18. 

    Thank you and Best Regards,

    Luis

    Precision Amplifiers Applications

  • 0 in reply to Luis Chioye
    Intellectual 440 points

    Hi Luis,

    Thank you very much for helping me, reviewing my design in detail and providing very valuable information/suggestions. Your suggestions helped me a lot.

    Best regards,

    Ahmet

  • 0 in reply to Ahmet Inan
    TI__Guru 52106 points

    Hi Ahmet,

    Thank you,

    Best Regards,

    Luis