Intel 8XC196MH, 8XC196MD, 8XC196MC manual Suggested A/D Input Circuit

8XC196MC, MD, MH USER’S MANUAL

12.6.1.2Suggested A/D Input Circuit

The suggested A/D input circuit shown in Figure 12-8 provides limited protection against over- voltage conditions on the analog input. Should the input voltage be driven significantly below ANGND or above VREF, diode D2 or D1 will forward bias at about 0.8 volts. The device’s input protection begins to turn on at approximately 0.5 volts beyond ANGND or VREF. The 270Ω re- sistor limits the current input to the analog input pin to a safe value, less than 1 mA.

NOTE

Driving any analog input more than 0.5 volts beyond ANGND or VREF begins 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.

	VREF
	VREF
D1	270Ω
100Ω
	ACHx
(Optional)
D2	0.005µF
	ANGND
	8XC196
	Device
	ANGND
	A0082-03

Figure 12-8. Suggested A/D Input Circuit

12.6.1.3Analog 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 VSS pin as close to the device as possible, using a minimum trace length. In a noisy environment, we highly recommend the use of a sepa- rate analog ground plane that connects to VSS at a single point as close to the device as possible. IREF may vary between 2 mA and 5 mA during a conversion. To minimize the effect of this fluc- tuation, mount a 1.0 µF ceramic or tantalum bypass capacitor between V REF and ANGND, as close to the device as possible.

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