Suggested A/D Input Circuit; Analog Ground And Reference Voltages - Intel 8XC196NT User Manual

11.6.1.2

Suggested A/D Input Circuit

The suggested A/D input circuit shown in Figure 11-8 provides limited protection against over-
voltage conditions on the analog input. Should the input voltage be driven significantly below
ANGND or above V
REF
protection begins to turn on at approximately 0.5 volts beyond ANGND or V
sistor limits the current input to the analog input pin to a safe value, less than 1 mA.
Driving any analog input more than 0.5 volts beyond ANGND or V
to activate the input protection devices. This drives current into the internal
reference circuitry and substantially degrades the accuracy of A/D conversions
on all channels.
100Ω
(Optional)
11.6.1.3

Analog Ground and Reference Voltages

Reference supply levels strongly influence the absolute accuracy of the conversion. For this rea-
son, we recommend that you tie the ANGND pin to the V
using a minimum trace length. In a noisy environment, we highly recommend the use of a sepa-
rate analog ground plane that connects to V
I may vary between 2 mA and 5 mA during a conversion. To minimize the effect of this fluc-
REF
tuation, mount a 1.0 µF ceramic or tantalum bypass capacitor between V
close to the device as possible.
, diode D2 or D1 will forward bias at about 0.8 volts. The device's input
NOTE
V
REF
D1
270Ω
0.005µF
D2
Figure 11-8. Suggested A/D Input Circuit
SS
ANALOG-TO-DIGITAL CONVERTER
V
REF
ACH x
ANGND
ANGND
pin as close to the device as possible,
SS
at a single point as close to the device as possible.
. The 270Ω re-
REF
begins
REF
8XC196
Device
A0082-03
and ANGND, as
REF
11-13