Input Current
The converters’ analog inputs behave somewhat differ-
ently from conventional ADCs. The sampled data com-
parators take varying amounts of current from the input,
depending on the cycle they are in. The equivalent cir-
cuit of the converter is shown in Figure 9a. When the
conversion starts, AIN(n) is connected to the MS and
LS comparators. Thus, AIN(n) is connected to thirty-one
1pF capacitors.
To acquire the input signal in approximately 1µs, the input
capacitors must charge to the input voltage through the
on-resistance of the multiplexer (about 600Ω) and the
comparator’s analog switches (2kΩ to 5kΩ per compara-
tor). In addition, about 12pF of stray capacitance must be
charged. The input can be modeled as an equivalent RC
network shown in Figure 9b. As R
S
(source impedance)
increases, the capacitors take longer to charge.
Since the length of the input acquisition time is internal-
ly set, large source resistances (greater than 100Ω) will
cause settling errors. The output impedance of an op-
amp is its open-loop output impedance divided by the
loop gain at the frequency of interest. It is important
that the amplifier driving the converter input have suffi-
cient loop gain at approximately 1MHz to maintain low
output impedance.
Input Filtering
The transients in the analog input caused by the sam-
pled data comparators do not degrade the converter’s
performance, since the ADC does not “look” at the
input when these transients occur. The comparator’s
outputs track the input during the first 1µs of the con-
version, and are then latched. Therefore, at least 1µs
will be provided to charge the ADC’s input capaci-
tance. It is not necessary to filter these transients with
an external capacitor on the AIN terminals.
Sinusoidal Inputs
The MAX154/MAX158 can measure input signals with
slew rates as high as 157mV/µs to the rated specifications.
This means that the analog input frequency can be as
high as 10kHz without the aid of an external track/hold.
The maximum sampling rate is limited by the conversion
time (typical t
CRD
= 2µs) plus the time required between
conversions (t
p
= 500ns). It is calculated as:
f
MAX
=
1
=
1
=
400kHz
t
CRD
+ t
p
(2.0 + 0.5) µs
f
MAX
permits a maximum sampling rate of 50kHz per
channel when using the MAX158 and 100kHz per
channel when using the MAX154. These rates are well
above the Nyquist requirement of 20kHz sampling rate
for a 10kHz input bandwidth.
MAX154/MAX158
CMOS High-Speed 8-Bit ADCs with
Multiplexer and Reference
_______________________________________________________________________________________ 9
MAX154
MAX158
V
IN
GND
V
DD
REFOUT
V
REF
+
AIN
x
(+)
AIN
x
(-)
+5V
0.1µF47µF
0.01µF
V
REF
-
MAX154
MAX158
V
IN
GND
V
DD
V
REF
+
AIN
x
(+)
AIN
x
(-)
+5V
0.1µF
47µF
V
REF
-
MAX154
MAX158
V
IN
GND
* Current path must
still exist from
V
IN(-)
to Ground
V
DD
V
REF
+
AIN
x
(+)
AIN
x
(-)
*
+5V
2.5V
0.1µF
47µF
V
REF
-
Figure 8a. Internal Reference
Figure 8b. Power Supply as Reference
Figure 8c. Inputs Not Referenced to GND
