ADC0804LCWMX datasheet(21/48 Pages) TI

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SNOSBI1B – NOVEMBER 2009 – REVISED FEBRUARY 2013

rON of SW 1 and SW 2 . 5 kΩ

r=rON CSTRAY × 5 kΩ x 12 pF = 60 ns

Figure 49. Analog Input Impedance

The voltage on this capacitance is switched and will result in currents entering the VIN(+) input pin and leaving

the VIN(−) input which will depend on the analog differential input voltage levels. These current transients occur

at the leading edge of the internal clocks. They rapidly decay and do not cause errors as the on-chip comparator

is strobed at the end of the clock period.

Fault Mode

If the voltage source applied to the VIN(+) or VIN(−) pin exceeds the allowed operating range of VCC+50 mV, large

input currents can flow through a parasitic diode to the VCC pin. If these currents can exceed the 1 mA max

allowed spec, an external diode (1N914) should be added to bypass this current to the VCC pin (with the current

bypassed with this diode, the voltage at the VIN(+) pin can exceed the VCC voltage by the forward voltage of this

diode).

Input Bypass Capacitors

Bypass capacitors at the inputs will average these charges and cause a DC current to flow through the output

resistances of the analog signal sources. This charge pumping action is worse for continuous conversions with

the VIN(+) input voltage at full-scale. For continuous conversions with a 640 kHz clock frequency with the VIN(+)

input at 5V, this DC current is at a maximum of approximately 5 µA. Therefore, bypass capacitors should not be

used at the analog inputs or the VREF/2 pin for high resistance sources (> 1 kΩ). If input bypass capacitors are

necessary for noise filtering and high source resistance is desirable to minimize capacitor size, the detrimental

effects of the voltage drop across this input resistance, which is due to the average value of the input current,

can be eliminated with a full-scale adjustment while the given source resistor and input bypass capacitor are both

in place. This is possible because the average value of the input current is a precise linear function of the

differential input voltage.

Input Source Resistance

Large values of source resistance where an input bypass capacitor is not used, will not cause errors as the input

currents settle out prior to the comparison time. If a low pass filter is required in the system, use a low valued

series resistor (

≤ 1 kΩ) for a passive RC section or add an op amp RC active low pass filter. For low source

resistance applications, (

≤ 1 kΩ), a 0.1 μF bypass capacitor at the inputs will prevent noise pickup due to series

lead inductance of a long wire. A 100

Ω series resistor can be used to isolate this capacitor — both the R and C

are placed outside the feedback loop — from the output of an op amp, if used.

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