MAX154 Datasheet, PDF (9/12 Pages) Maxim Integrated Products – CMOS High-Speed 8-Bit ADCs with Multiplexer and Reference

CMOS High-Speed 8-Bit ADCs with
Multiplexer and Reference
AIN_x(+)
AIN_x(-)
+5V
0.1µF
47µF
0.01µF
Figure 8a. Internal Reference
V_IN
GND
V_DD
MAX154
MAX158
REFOUT
V_REF+
V_REF-
+5V
0.1µF
AIN_x(+)
AIN_x(-)
47µF
V_IN
GND
MAX154
V_{DD MAX158}
V_REF+
V_REF-
Figure 8b. Power Supply as Reference
* Current path must
still exist from
V_IN(-) to Ground
AIN_x(+)
+5V
0.1µF
47µF
2.5V
AIN_x(-)
*
V_IN
GND
V_DD MAX154
MAX158
V_REF+
V_REF-
Figure 8c. Inputs Not Referenced to GND
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.
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