Datasheet

ADC10662, ADC10664
www.ti.com SNAS076E –JUNE 1999–REVISED MARCH 2013
Table 1. Input Multiplexer Programming ADC10664
S
1
S
0
Channel
0 0 V
IN0
0 1 V
IN1
1 0 V
IN2
1 1 V
IN3
Table 2. Input Multiplexer Programming ADC10662
S
0
Channel
0 V
IN0
1 V
IN1
Mode 2
In Mode 2, also called “RD mode”, the S/H and RD pins are tied together. A conversion is initiated by pulling both
pins low. The A/D converter samples the input voltage and causes the coarse comparators to become active. An
internal timer then terminates the coarse conversion and begins the fine conversion. 470 ns (typical) after S/H
and RD are pulled low, INT goes low, indicating that the conversion is 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.
REFERENCE CONSIDERATIONS
The ADC10662 and ADC10664 each have two reference inputs. These inputs, V
REF+
and V
REF−
, are fully
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
REF−
= 0V, V
REF+
= V
CC
) for ratiometric applications, or they can be
connected to different voltages (as long as they are between ground and V
CC
) when other input spans are
required.
Reducing the overall V
REF
span to less than 5V increases the sensitivity of the converter (e.g., if V
REF
= 2V, then
1 LSB = 1.953 mV). Note, however, that linearity and offset errors become larger when lower reference voltages
are used. See Typical Performance Characteristics for more information. For this reason, reference voltages less
than 2V are not recommended.
In most applications, V
REF−
will simply be connected to 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 ADC10662
and ADC10664. V
REF−
can be connected to a voltage other than ground as long as the voltage source connected
to this pin is capable of sinking the converter's reference current (12.5 mA Max @ V
REF
= 5V). If V
REF−
is
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 400Ω, 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.
THE ANALOG INPUT
The ADC10662 and ADC10664 sample the analog input voltage once every conversion cycle. When this
happens, the input is briefly connected to an impedance approximately equal to 600Ω 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 500Ω 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/600Ω load. Any ringing or voltage shifts at the op-amp's output during the sampling period can result in
conversion errors.
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