MAX189CEPA(2012) Ver la hoja de datos (PDF) - Maxim Integrated

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MAX189CEPA
(Rev.:2012)

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

Maxim Integrated 

MAX187/ MAX189

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

Input Protection

Internal protection diodes that clamp the analog input

allow the input to swing from GND - 0.3V to VDD + 0.3V

without damage. However, for accurate conversions near

full scale, the input must not exceed VDD by more 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 practical choices.

The allowed input frequency range is limited by the

75ksps sample rate of the MAX187/MAX189. Therefore,

the maximum sinusoidal input frequency allowed is

37.5kHz. Higher-frequency signals cause aliasing prob-

lems 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 buff-

ered temperature-compensated bandgap diode, laser-

trimmed to +4.096V Q0.5%. Its output is connected to

REF and also drives the internal DAC. The output can be

used as a reference voltage source for other components

and can source up to 0.6mA. Decouple REF with a 4.7FF

capacitor. The internal reference is enabled by pulling

the SHDN pin high. Letting SHDN float disables the inter-

nal reference, which allows the use of an external refer-

ence, 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

within the voltage range +2.5V to VDD to achieve speci-

fied accuracy. The minimum input impedance is 12kI for

DC currents. During conversion, the external reference

must be able to deliver up to 350FA DC load current and

have an output impedance of 10I or less. The recom-

mended minimum value for the bypass capacitor is 0.1FF.

If the reference has higher output impedance or is noisy,

bypass it close to the REF pin with a 4.7FF capacitor.

COMPLETE CONVERSION SEQUENCE

CS

SHDN

tWAKE

DOUT

CONVERSION 1

POWERED UP

POWERED DOWN

CONVERSION 1

POWERED UP

Figure 5. MAX187/MAX189 Shutdown Sequence

Maxim Integrated

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