![](/images/new-backgrounds/1166768/16676871x1.webp)
Simplified Analog Input Analysis
Using the equivalent circuit in Figure
The capacitance charging voltage is given by
Vc = Vs (1 − e−tc RtCi )
where
Rt = Rs + ri
The final voltage to ½ LSB is given by
Vc (12 LSB) = Vs − (Vs
2048)
Equating equation 1 to equation 2 and solving for time tc gives
Vs − (Vs 2048) = Vs (1 − e−tc RtCi )
and
tc (12 LSB) = Rt × Ci × ln(2048)
(1)
(2)
(3)
(4)
Therefore, with the values given the time for the analog input signal to settle is
tc (1 2 LSB) = ( Rs + 1kΩ) × 60pF × ln(2048) | (5) |
This time must be less than the converter sample time shown in the timing diagrams.
VI = Input Voltage at A0 - A10
VS = External Driving Source Voltage RS = Source Resistance
ri = Input Resistance
Ci = Equivalent Input Capacitance *Driving source requirements:
∙Noise and distortion for the source must be equivalent to the resolution of the converter.
∙RS must be real at the input frequency.
Appendix B | 485SDA103798 Manual |
B&B Electronics
PH (815)