Rainbow-electronics MAX1197 Bedienungsanleitung PDF herunterladen (Seite 17)

MAX1197

Dual, 8-Bit, 60Msps, 3V, Low-Power ADC with

Internal Reference and Parallel Outputs

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cutoff frequency) to the MAX4250, which buffers the

reference before its output is applied to a second 10Hz

lowpass filter. The MAX4250 provides a low offset volt-

age (for high gain accuracy) and a low noise level. The

passive 10Hz filter following the buffer attenuates noise

produced in the voltage reference and buffer stages.

This filtered noise density, which decreases for higher

frequencies, meets the noise levels specified for preci-

sion ADC operation.

Unbuffered External Reference Drives

Multiple ADCs

Connecting each REFIN to analog ground disables the

internal reference of each device, allowing the internal

reference ladders to be driven directly by a set of

external reference sources. Followed by a 10Hz low-

pass filter and precision voltage divider, the MAX6066

generates a DC level of 2.500V. The buffered outputs

of this divider are set to 2.0V, 1.5V, and 1.0V, with an

accuracy that depends on the tolerance of the divider

resistors. These three voltages are buffered by the

MAX4252, which provides low noise and low DC offset.

The individual voltage followers are connected to 10Hz

lowpass filters, which filter both the reference voltage

and amplifier noise to a level of 3nV/√Hz. The 2.0V and

1.0V reference voltages set the differential full-scale

range of the associated ADCs at 2V

. The 2.0V and

1.0V buffers drive the ADC’s internal ladder resistances

between them.

Note that the common power supply for all active com-

ponents removes any concern regarding power-supply

sequencing when powering up or down. With the out-

puts of the MAX4252 matching better than 0.1%, the

buffers and subsequent lowpass filters can be replicat-

ed to support as many as 32 ADCs. For applications

that require more than 32 matched ADCs, a voltage

reference and divider string common to all converters

is highly recommended.

Typical QAM Demodulation Application

A frequently used modulation technique in digital com-

munications applications is quadrature amplitude

MAX4250

MAX6062

16.2k

Ω

162

Ω

3.3V

10Hz LOWPASS

FILTER

10Hz LOWPASS

FILTER

REFOUT

REFP

REFIN

MAX1197

N = 1

REFN

29N.C.

2.048V

N.C.

COM

REFOUT

NOTE: ONE FRONT-END REFERENCE CIRCUIT DESIGN MAY BE USED WITH UP TO 1000 ADCs.

REFP

REFIN

MAX1197

N = 1000

REFN

COM

0.1

3.3V

0.1

F0.1

0.1

2.2

10V

0.1

F0.1

0.1

100

0.1

Figure 8. External Buffered (MAX4250) Reference Drive Using a MAX6062 Bandgap Reference

1 2 ... 12 13 14 15 16 17 18 19 20 21 22

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