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Functional Description (Continued)

2.2 Analog Differential Voltage Inputs and

Common-Mode Rejection

This A/D has additional applications flexibility due to the ana-

log differential voltage input. The V

IN(−) input (pin 7) can be

used to automatically subtract a fixed voltage value from the

input reading (tare correction). This is also useful in 4 mA–20

mA current loop conversion. In addition, common-mode

noise can be reduced by use of the differential input.

The time interval between sampling V

IN(+) and VIN(−) is 4-

⁄2

clock periods. The maximum error voltage due to this slight

time difference between the input voltage samples is given

by:

where:

∆V

e is the error voltage due to sampling delay

P is the peak value of the common-mode voltage

cm is the common-mode frequency

As an example, to keep this error to 1⁄4 LSB (

∼5 mV) when

operating with a 60 Hz common-mode frequency, f

cm, and

using a 640 kHz A/D clock, f

CLK, would allow a peak value of

the common-mode voltage, V

P, which is given by:

which gives

P≅1.9V.

The allowed range of analog input voltages usually places

more severe restrictions on input common-mode noise lev-

els.

An analog input voltage with a reduced span and a relatively

large zero offset can be handled easily by making use of the

differential input (see section 2.4 Reference Voltage).

2.3 Analog Inputs

2.3 1 Input Current

Normal Mode

Due to the internal switching action, displacement currents

will flow at the analog inputs. This is due to on-chip stray ca-

pacitance to ground as shown in

Figure 5.

The voltage on this capacitance is switched and will result in

currents entering the V

IN(+) input pin and leaving the VIN(−)

input which will depend on the analog differential input volt-

age 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 V

IN(+) or VIN(−) pin ex-

ceeds the allowed operating range of V

CC+50 mV, large in-

put currents can flow through a parasitic diode to the V

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 V

CC pin (with the current bypassed with

this diode, the voltage at the V

IN(+) pin can exceed the VCC

voltage by the forward voltage of this diode).

2.3.2 Input Bypass Capacitors

Bypass capacitors at the inputs will average these charges

and cause a DC current to flow through the output resis-

tances of the analog signal sources. This charge pumping

action is worse for continuous conversions with the V

IN(+) in-

put voltage at full-scale. For continuous conversions with a

640 kHz clock frequency with the V

IN(+) 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 (> 1kΩ). 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 resis-

tance, 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 in-

put current is a precise linear function of the differential input

voltage.

2.3.3 Input Source Resistance

Large values of source resistance where an input bypass ca-

pacitor 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

(

≤ 1kΩ) for a passive RC section or add an op amp RC ac-

tive low pass filter. For low source resistance applications,

(

≤ 1kΩ), a 0.1 µF bypass capacitor at the inputs will prevent

noise pickup due to series lead inductance of a long wire. A

DS005671-14

rON of SW 1 and SW 2 ≅ 5kΩ

r=rON CSTRAY ≅ 5kΩ x12pF = 60 ns

FIGURE 5. Analog Input Impedance

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