completed. Approximately 20 ns later the data appearing on

the TRI-STATE output pins will be valid. Note that data will

appear on these pins throughout the conversion, but until

INT goes low the data at the output pins will be the result of

the previous conversion.

2.0 REFERENCE CONSIDERATIONS

The ADC10461, ADC10462, and ADC10464 each have two

reference inputs. These inputs, V

differential and define the zero to full-scale range of the input

signal. The reference inputs can be connected to span the

entire supply voltage range (V

ratiometric applications, or they can be connected to differ-

ent voltages (as long as they are between ground and V

when other input spans are required.

Reducing the overall V

the sensitivity of the converter (e.g., if V

= 1.953 mV). Note, however, that linearity and offset errors

become larger when lower reference voltages are used. See

the Typical Performance Curves for more information. For

this reason, reference voltages less than 2V are not recom-

mended.

In most applications, V

ground, but it is often useful to have an input span that is

offset from ground. This situation is easily accommodated by

the

reference

configuration

used

in

the ADC10461,

ADC10462, and ADC10464. V

voltage other than ground as long as the voltage source

connected to this pin is capable of sinking the converter’s

connected to a voltage other than ground, bypass it with

multiple capacitors.

Since the resistance between the two reference inputs can

be as low as 400

Ω, the voltage source driving the reference

inputs should have low output impedance. Any noise on

either reference input is a potential cause of conversion

errors, so each of these pins must be supplied with a clean,

low noise voltage source. Each reference pin should be

bypassed with a 10 µF tantalum and a 0.1 µF ceramic.

3.0 THE ANALOG INPUT

The ADC10461, ADC10462, and ADC10464 sample the

analog input voltage once every conversion cycle. When this

happens, the input is briefly connected to an impedance

approximately equal to 600

Ω in series with 35 pF. Short-

duration current spikes can be observed at the analog input

during normal operation. These spikes are normal and do

not degrade the converter’s performance.

Large source impedances can slow the charging of the

sampling capacitors and degrade conversion accuracy.

Therefore, only signal sources with output impedances less

than 500

Ω should be used if rated accuracy is to be

achieved at the minimum sample time (250 ns maximum). If

the sampling time is increased, the source impedance can

be larger. If a signal source has a high output impedance, its

output should be buffered with an operational amplifier. The

operational amplifier’s output should be well-behaved when

driving a switched 35 pF/600

Ω load. Any ringing or voltage

shifts at the op amp’s output during the sampling period can

result in conversion errors.

Correct conversion results will be obtained for input voltages

greater than GND − 50 mV and less than V

allow the signal source to drive the analog input pin beyond

the Absolute Maximum Rating. If an analog input pin is

forced beyond these voltages, the current flowing through

the pin should be limited to 5 mA or less to avoid permanent

damage to the IC. The sum of all the overdrive currents into

all pins must be less than the Absolute Maximum Rating for

Package Input Current. When the input signal is expected to

extend beyond this limit, an input protection scheme should

be used. A simple input protection network using diodes and

resistors is shown in Figure 4. Note the multiple bypass

capacitors on the reference and power supply pins. If V

REF−

is not grounded, it should also be bypassed to analog ground

using multiple capacitors (see 5.0 “Power Supply Consider-

ations”). AGND and DGND should be at the same potential.

V

normally left open, but optional “speedup” resistor R

be used to reduce the conversion time.

01110814

FIGURE 4. Typical Connection

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