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ADS130E08: valid analog input range

DDdoor
Expert 2435 points
Part Number: ADS130E08
Other Parts Discussed in Thread: ADS124S08

Hello,

Please tell me about ADS130E08.

This device can be used as fully differential or pseudo differential,
Customers use it as pseudo differential.
INxN inputs 1.65V. And INxP is input from 0V to 3.3V.
AVDD = 3.3V, AVSS = 0V, VREF = 3.3V (Vrefp-Vrefn = 3.3V-0V)

I confirmed the data sheet. The datasheet shows the analog input as ± VREF / gain. And its VREF is AVSS-0.3V to AVDD + 0.3V.
In this context, the analog input is in the 0V to 3.3V range.
It looks like you can enter it.

on the other hand
I confirmed the contents of the Input Common-Mode Range in the data sheet.
When AVDD = 3.3 V, AVSS = 0 V, and Vmmax_diff is 3.3 V
Inequality does not hold.

AVDD-0.2-(GainVmax_diff / 2)> CM> AVSS + 0.2 + (GainVmax_diff)

Both full differential and pseudo differential
Is the valid analog input range from 0.2V to 3.1V?

Best regards,
DDdoor

  • 0
    TI__Guru* 98115 points

    DDdoor,

    In the case that you describe, the valid input analog range is 0.2V to 3.1V. I'll explain the specifications that you've mentioned and show what they mean.

    The first thing that you mention is the reference input and the specification for the Absolute Maximum Ratings (page 2). This is from the table:

    Note that this is from the Absolute Maximum Ratings table. This means that if you exceed these values, you may damage the device. It does not mean that you would want to operate this at these maximum voltages.

    Normally we look to the Recommended Operating Conditions table for the typical operation for newer datasheets. In this older datasheet, you find it in the Electrical Characteristics table:

    Here we say that the reference input can be operated from AVSS which is the typical negative input, to 2.5V. The typical reference input is 2.5V or 4V depending on the supply voltage. It doesn't say specifically, but the input can be from AVSS to AVDD.

    In the same table, there is a range for the analog inputs. You can see them here:

    This has a heading for the differential voltage and for the common-mode input voltage. The differential input voltage is ±VREF/Gain. This means that the ADC will measure inputs that are this large. VREF/Gain would be the maximum that the ADC could measure and -VREF/Gain would be the minimum. This accounts for the ADC measurement based on the reference, but it does not consider the limitations of the input voltage based on the supply and PGA. The second listing for the common-mode input voltage considers those two effects. Going to the input common-mode range section, the input common-mode range is limited to:

    It's not entirely clear in the description, what this is showing is that the PGA has limitations and your input must be within a certain range because the PGA is limited. The PGA is formed like the front end of an instrumentation amplifier. Because of this PGA input is limited by the amplifer range of operation. The limitation can come from the input range of the op-amps or from the output range of the op-amp. Because the input and output of this PGA must have the same common-mode voltage, the input common-mode is more limited with higher gain.

    Again, the ADS130E08 is an older datasheet. There a similar but better explanation in some of our newer datasheets. Look at the ADS124S08 datasheet in the Low-Noise Programmable Gain Amplifier section.

    The PGA is similar, but the explanation of the common-mode input range is more clear.

    Regardless, you are correct, if the gain is 1, the absolute input voltages would be limited to 0.2V to 3.1V. If pseudo-differential inputs are used, the differential inputs would be ±1.45V because of the PGA range. I would guess that the output voltage of the PGA would get close to the supplies, but it is very likely that for outputs within 0.2V of either supply, the measurement would be non-linear.

    Joseph Wu