JAJSFU8D November 2017 – June 2024 ADS8166 , ADS8167 , ADS8168
PRODUCTION DATA
The ADS816x enables using a common amplifier to drive the ADC inputs as shown in Figure 8-1. This configuration improves offset error mismatch between the analog inputs as the offset error of the amplifier is common to all the analog input channels.
When connecting the sensor directly to the input of the ADS816x, the maximum switching speed of the multiplexer is limited by multiplexer on-resistance and parasitic capacitance. Figure 8-2 illustrates the source resistance (RS0, RS1, and so forth), multiplexer impedance (RMUX), multiplexer capacitance (CMUX), op amp input capacitance (COPA), and the stray PCB capacitance at the output of the multiplexer (CSTRAY). In this example, the total output capacitance is the combination of the multiplexer output capacitance, the op amp input capacitance, and the stray capacitance (CMUX + COPA + CSTRAY) = 15pF. When switching to a channel, make sure this capacitance is charged to the sensor output voltage with the source resistance and the multiplexer resistance (RS0 + RMUX).
Equation 2 estimates the number of time constants required for N bits of settling. For this example, to achieve 16-bit settling, 11.09 time constants are required. Thus, as computed in Equation 3 and Equation 4, for channel 0 the required settling time is 167ns.
When operating at 1MSPS in either manual mode, auto sequence mode, or custom channel sequencing mode, a 900ns settling time is available at the analog inputs of the multiplexer. Using Equation 4, the maximum sensor output impedance for a direct connection is 5.4kΩ.
Figure 8-3 shows that the multiplexer inputs are driven using an amplifier. The multiplexer outputs are connected to the ADC inputs directly. For best distortion performance, place an amplifier between the multiplexer and the ADC as shown in Figure 8-2.