ADS8681EVM-PDK: Analog Input Circuit

Hi Cafain,

Answer 1:  This is not a problem. ADS8681 datasheet has mentioned that "If the analog input pin (AIN_P) to the device is left floating, the output of the ADC corresponds to an internal biasing voltage. The output from the ADC must be considered as invalid if the device is operated with floating input pins. This condition does not cause any damage to the device, which becomes fully functional when a valid input voltage is applied to the pins" in page 24 (7.3.1 Analog Input Structure). Also, the Electrical Characteristics table in the datasheet as below shows this information about this bias voltage(e.g. (~2.5V bias voltage for +/-3Vref range).

ADS8681 has an integrated analog front-end (AFE), the simplified input circuit is shown with a bias voltage(~2.5V for +/-3Vref range) in Figure 47 also as below, the input signal is biased to the appropriate common-mode voltage level for proper operation of internal ADC core. When there is an available input signal on ADC input, the available signal will dominate it, if the input is floating and no any signal, the bias voltage will be observed, the converted voltage/code may be different for different front-end RC filter if a pull down resistor is used before filter. Since this ADC supports true bipolar input signals using a single +5V power supply, the internal biasing cannot be done to provide a zero offset at ADC output when the inputs are left floating. One of the advantages of this input stage is that it offers a 1-MOhm constant resistive input impedance.  This makes the inputs relatively easy to drive (in most cases) without additional driver amplifiers or buffers. General switched-capacitor input structure ADC needs a lot of cautions to design front-end amplifier and RC filter, any improper amplifier and RC selection will lead to large error to settle the signal on internal sampling capacitors and will get worse performance result, but the front-end design for this ADS8681 with such integrated AFE will be much easier, and the ADC can be driven directly without any external amplifier because the AFE and ADC driver have been integrated into these ADC.

To emphasize again, this bias voltage was known when the ADC was designed for this structure and these benefits. This is very common for the ADCs with such integrated AFE and input structure

Answer 2:  Please see the simplified input circuit in Figure 47 also as above.

Answer 3:  It is better to show your schematic here. As mentioned in answer 1, this ADC does not require a specific driving circuit to settle the signal and drive it. Hence, your 470ohm resister should be fine if it is inserted in series with the ADC input and my understanding is correct.

Answer 4:  I would like to check on the EVM but I'm on a business trip. I will try to check with peers and get it back to you. Please allow me for a delay.

Best regards

Dale

Hi, Dale-san