+5V, Low-Power, 12-Bit Serial ADCs

10

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COMPLETE CONVERSION SEQUENCE

POWERED UP

POWERED DOWN

POWERED UP

CONVERSION 0

CONVERSION 1

DOUT

CS

SHDN

Input Bandwidth

The ADCs’ input tracking circuitry has a 4.5MHz small-

signal bandwidth, and an 8V/µs slew rate. It is possible

to digitize high-speed transient events and measure

periodic signals with bandwidths exceeding the ADC's

sampling rate by using undersampling techniques. To

avoid aliasing of unwanted high-frequency signals into

the frequency band of interest, an anti-alias filter is rec-

ommended. See the MAX274/MAX275 continuous-time

filters data sheet.

Input Protection

Internal protection diodes that clamp the analog input

without damage. However, for accurate conversions

than 50mV, or be lower than GND by 50mV.

If the analog input exceeds the supplies by more than

50mV beyond the supplies, limit the input current to

2mA, since larger currents degrade conversion

accuracy.

Driving the Analog Input

The input lines to AIN and GND should be kept as short

as possible to minimize noise pickup. Shield longer

leads. Also see the

Input Protection section.

Because the MAX187/MAX189 incorporate a T/H, the

drive requirements of the op amp driving AIN are less

stringent than those for a successive-approximation

ADC without a T/H. The typical input capacitance is

16pF. The amplifier bandwidth should be sufficient to

handle the frequency of the input signal. The MAX400

and OP07 work well at lower frequencies. For higher-

frequency operation, the MAX427 and OP27 are practi-

cal choices. The allowed input frequency range is limit-

ed by the 75ksps sample rate of the MAX187/MAX189.

Therefore, the maximum sinusoidal input frequency

allowed is 37.5kHz. Higher-frequency signals cause

aliasing problems unless undersampling techniques

are used.

Reference

The MAX187 can be used with an internal or external ref-

erence, while the MAX189 requires an external reference.

Internal Reference

The MAX187 has an on-chip reference with a buffered

temperature-compensated bandgap diode, laser-

trimmed to +4.096V ±0.5%. Its output is connected to

REF and also drives the internal DAC. The output can

be used as a reference voltage source for other com-

ponents and can source up to 0.6mA. Decouple REF

with a 4.7µF capacitor. The internal reference is

enabled by pulling the

SHDN pin high. Letting SHDN

float disables the internal reference, which allows the

use of an external reference, as described in the

External Reference section.

External Reference

The MAX189 operates with an external reference at the

REF pin. To use the MAX187 with an external reference,

disable the internal reference by letting

SHDN float. Stay

fied accuracy. The minimum input impedance is 12k

Ω

for DC currents. During conversion, the external refer-

ence must be able to deliver up to 350µA DC load cur-

rent and have an output impedance of 10

Ω or less. The

recommended minimum value for the bypass capacitor

is 0.1µF. If the reference has higher output impedance

or is noisy, bypass it close to the REF pin with a 4.7µF

capacitor.

Figure 5. MAX187/MAX189 Shutdown Sequence