TS7003
process, the input side of C
HOLD
is switched back to
AIN so as to be charged to the input signal again.
An A C’s acquisition time is function of how fast its
input capacitance can be charged. If an input signal’s
driving-point source impedance is high, the
acquisition time is lengthened and more time must be
allowed between conversions. The acquisition time
(t
ACQ
) is the maximum time the ADC requires to
acquire the signal and is also the minimum time
needed for the signal to be acquired. The TS7003’s
acquisition time is calculated from the following
expression:
t
ACQ
= 9 x (R
S
+ R
IN
) x 10pF
where R
IN
= 100Ω (the TS7003’s internal track/hold
switch resistance), R
S
= the input signal’s source
impedance, and t
ACQ
is never less than 625ns.
Because of the input structure of the TS7003,
sources with output impedances of 1kΩ or less do not
affect significantly the AC performance of the
TS7003. The TS7003 can still be used in applications
where the source impedance is higher so long as a
0.01μF capacitor is connected between the analog
input and G . Limiting the A C’s input signal
bandwidth, the use of an external, input capacitor
forms an RC filter with the input’s source impedance.
Input Bandwidth Considerations
Since the TS7003’s input track-and-hold circuit
exhibits a 10 MHz small-signal bandwidth, it is
possible to measure periodic signals and to digitize
high-speed transient events with signal bandwidths
higher than the TS7003’s sampling rate by using
undersampling techniques. To avoid the aliasing of
high-frequency signals into the frequency band of
interest, the use of external anti-alias filter circuits
(discrete or integrated) is recommended. The time
constant of the external anti-alias filter should be set
so as not to interfere with the desired signal
bandwidth.
Analog Input Protection
The TS7003 incorporates internal protection diodes
that clamp the analog input between V
DD
and GND.
These internal protection diodes allow the AIN pin to
swing from GND - 0.3V to V
DD
+ 0.3V without causing
damage to the TS7003. However, for accurate
conversions near full scale, the input signal must not
exceed V
DD
by more than 50mV or be lower than
GND by 50mV.
If the analog inputs can exceed 50mV beyond the
supplies, then the current in the forward-biased
protection diodes should be limited to less than
2mA since large fault currents can affect
conversion results.
Internal Reference Considerations
The TS7003 has an internal voltage reference that is
factory-trimmed to 2.5V. The internal reference output
is connected to the REF pin and is also connected to
the A C’s internal C AC. The REF output can be
used as a reference voltage source for other
components external to the ADC and can source up
to 750μA. To maintain conversion accuracy to within
1 LSB, a 4.7μF capacitor from the REF pin to G
is
recommended. While larger-valued capacitors can be
used to further reduce reference wide-band noise,
larger capacitor values can increase the TS7003’s
wake-up time when exiting from shutdown mode (see
the “Using SH
to Reduce Operating Supply
Current” section for more information). When in
shutdown
(that
is,
when
SH
= 0), the TS7003’s internal 2.5-V reference is
disabled.
Serial Digital Interface
Initialization
Conversion
after
Power-Up
and
Starting
a
If the SH
pin is not driven low upon an initial, cold-
start condition, it may take up to 2.5ms for a fully-
discharged 4.7μF reference bypass capacitor to
provide adequate charge for specified conversion
accuracy. As a result, conversions should not be
initiated during this reference capacitor charge-up
delay. To initiate a conversion, the CS pin is toggled
(or driven) low. At the CS’s falling edge, the TS7003’s
internal track-and-hold is placed in hold mode and a
conversion is initiated. Data can then be transferred
out of the ADC using an external serial clock.
TS7003DS r1p0
Page 9
RTFDS
TOUCHSTONE [ TOUCHSTONE SEMICONDUCTOR INC ]
