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PGA204: Input Common-Mode Range for ±5V Supply

Emre Oguz
Prodigy 10 points
Part Number: PGA204
Other Parts Discussed in Thread: ADS131M08, PGA855, INA851

Tool/software:

Hello,

Datasheet of the PGA204 says that it operates with power supplies as low as ±4.5V, allowing use in battery operated systems. But the specification is given for ±15V supply.

Is it a good idea to operate PGA204 with ±5V supply? What will be the input common-mode range for ±5V supply?

Best regards,

Emre Oguz

  • 0
    TI__Mastermind 22332 points

    Hello Emre, 

    Thanks for posting on e2e, the PGA204 is from 1993 and doesn't have the level of detail of more modern programmable gain instrumentation amplifier datasheets. 

    To answer your question, the input common-mode voltage range is given in the context of the supplies used: 

    The supplies provided are ±15V, and the typical input-common mode range is ±12.7V, so it can be interpreted as (V-) + 2.3V to (V+) - 2.3V. 
    If the power supplies are as low as ±4.5V, then the expected common-mode voltage is ±2.2V. 

    Note that the output is also limited by the power supply headroom: 

    What is your expected input voltage range, desired output swing, and available power supplies in your system? I can help recommend another device if this one doesn't provide a solution. 

    All the best,
    Carolina 

  • 0 in reply to Caro Walter
    Prodigy 10 points

    Hello Caro,

    Thanks for the explanation. This means I will get ±2.7V input-common mode range for ±5V, as I understand.

    I am working on the signal chain between a resistor divider network and the ADC. After the resistor divider voltages change between 1mV to a few volts AC, 50Hz or 60Hz. You can consider the circuit as a precision AC millivoltmeter. Accuracy levels will be under 0.1%.

    I was planning to use ADS131M08, that's why I am trying to stay at a lower power supply voltage levels. Probably I will need 1000 to 4000 gain levels for the mV range. If possible I want to keep the signal chain fully differential until the ADC.

    I will be grateful if you have any solution with a new generation PGA.

    Best regards,

    Emre Oguz

  • +1 in reply to Emre Oguz
    TI__Mastermind 22332 points

    Hello Emre, 

    For a fully differential signal chain until the ADC, I recommend the PGA855. There is also an instrumentation amplifier version, INA851. 

    The PGA855 is limited to the following gains: 1/8. 1/4, 1/2, 1, 2, 4, 8, and 16. The INA851 provides gains from 1/5 to 10,000 using an external gain setting resistor.

    The benefit of the gain setting resistor being integrated (PGA855) is that all the resistors inside the device all drift the same way and it is easier to achieve higher accuracy across temperature. To have the same effect with the INA851 it will require a low PPM resistor. The PGA855 also allows the gain to be changed real time which is possible with the INA851 if you use a mux. 

    If you would like to move forward with the PGA855, I recommend utilizing the I/O Calculator on the product page: https://www.ti.com/tool/download/PGA85X-INPUT-OUTPUT-RANGE-DESIGN-CALC

    In this next scenario I made some assumptions about the ADS131M08, AVDD = 3V, AGND = 0V. Therefore, here is the analog inputs limitation: 

    Since the range into the PGA855 will be between 1mV and a few volts, we will have to use the highest gain available (PGA855) which is 16V/V for 1mV. If we are using the highest gain inside the ADC (128 V/V), then the available input range of the ADC is, VAINxP/N = 1.2V to -1.3V. 

    This can be achieved if the PGA855 is setup the following way: VS = LVS = ±5V, VOCM = 0V. The expected output voltage of the PGA855 is as follows, I have marked the ADC input limits. 

    As you can see after ~ ±150mV input differential voltage, either the ADC gain of 128V/V would be decreased, or the PGA gain of 16V/V would need to be decreased. 

    Either way, the combined gain in the mV range is 16V/V x 128V/V = 2048 V/V.

    If you would like I can double check the other extreme (a few volts). 

    In summary, I recommend the PGA855.
    If you are more interested in the flexibility provided by the INA851, please let me know and I will transfer this thread to my INA colleagues. 

    All the best,
    Carolina