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 (2k

Ω to 5kΩ per compara-

tor). In addition, about 12pF of stray capacitance must be

charged. The input can be modeled as an equivalent RC

increases, the capacitors 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 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

1

=

1

= 400kHz

(2.0 + 0.5) µs

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.

CMOS High-Speed 8-Bit ADCs with

Multiplexer and Reference

_______________________________________________________________________________________

9

MAX154

MAX158

GND

REFOUT

+5V

0.1

µF47µF

0.01

µF

MAX154

MAX158

GND

+5V

0.1

µF

47

µF

MAX154

MAX158

GND

* Current path must

still exist from

*

+5V

2.5V

0.1

µF

47

µF

Figure 8a. Internal Reference

Figure 8b. Power Supply as Reference

Figure 8c. Inputs Not Referenced to GND