Dear team,
My customer is using ADS8688 as V/I measurement in PLC application. When they plan to expand the scale range of the input using below circuit. they measured the AIN_5P and AIN_5GND with 2.2V and 1.2V value although there is no input on the AIN_5. Is it because of the floating of AIN_5 cause this happen? Since if AIN_5 input 0V , it is normal 0.
Leave the AIN5 floating and measure AIN__5P to AGND is 2.2V; measuring AIN_5GND to AGND is 1.2V. Customer want to read the value to be 0 when input connected nothing(Floating). How should they put a pull down resistor if leave AIN5 floating and measure AIN_to AIN_GND is zero?
Hi Zoe,
Ryan started PTO this afternoon. I'm secondary technical support for this device. Please see my comments below:
The 2.2V the customer measured is correct as they used ±2.5Vref input range of ADC. The ADS8688 family including ADS8668 device incorporates an analog front-end circuit consisting of a Programmable Gain Amplifier (PGA) with 1MOhm input resistors that is shown on Figure 67 of the datasheet as below. One of the advantages of this input stage is to offer a 1MOhm constant resistive input impedance. This makes the inputs relatively easy to drive without requiring additional driving amplifiers or buffers in most applications.
Since the ADC supports true bipolar input signals with a single +5V power supply, a bias voltage is required and expected because the front-end signal conditioning on each analog input channel is biased internally to the appropriate common-mode voltage level for proper operation of internal ADC. Since the internal PGA input is biased at the required common-mode voltage level, the device can not generate zero ADC conversion result when the input is left floating. The voltage that can be seen is different under input floating condition if the input range is different, see the highlight in red in the following table of input leakage current of the ADS8688 datasheet:
When an analog signal is connected and available on ADC's analog input pin, this internal bias voltage will not be seen in ADC's output, which means that the internal bias voltage does not have any impact to normal analog signal conversion and the conversion code.
Inserting an amplifier is an ideal solution if this is a real concern when the input is floating, however adding a pull-down resistor is a simple solution and is normally used by many customers. In a normal design with small value resistor of external RC filter, the pull-down resistor can be inserted between the positive input and ground prior to the external RC filter, it forms e a resistor divider with 1MOhm input impedance, the smaller value of pull-down resistor, the lower voltage that can be seen by the ADC when the input is floating. However, that solution will not work in this scaling design with a large value of external series resistor (3MOhm). One possible solution is to remove the 3MOhm on the negative input and insert the pull-down resistor between AIN_xP and AIN_xGND pin, but the RC filter will not be a balanced input filter and it may have an additional offset because of leakage current when it flows through the 3MOhm resistor on the positive input, a calibration may be needed depending on the customer's accuracy requirement.
Let me know if an additional explanation or communication is expected by the customer.
Thanks®ards,
Dale
