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

600

Ω) and the comparator’s analog switches (2kΩ to

5k

Ω per comparator). In addition, about 12pF of stray

capacitance must be charged. The input can be mod-

eled as an equivalent RC network shown in Figure 9b.

take longer to charge.

Since the length of the input acquisition time is internal-

ly set, large source resistances (greater than 100

Ω) 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 and MX7824/MX7828 can mea-

sure 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

500ns). It is calculated as:

1

=

1

= 400kHz

(2.0 + 0.5) µs

CMOS, High-Speed, 8-Bit ADCs

with Multiplexer

_______________________________________________________________________________________

9

MAX154

MAX158

GND

REF OUT

+5V

0.1

µF47µF

0.01

µF

MX7824

MX7828

MX584

GND

+5V

2.5V

0.1

µF

47

µF

MAX154

MAX158

MX7824

MX7828

GND

+5V

0.1

µF

47

µF

MAX154

MAX158

MX7824

MX7828

GND

* Current path must

still exist from

+5V

2.5V

0.1

µF47µF

*

Figure 8a. Internal Reference (MAX154/MAX158 only)

Figure 8b. External Reference +2.5V Full-Scale

Figure 8c. Power Supply as Reference

Figure 8d. Inputs Not Referenced to GND