Unbuffered external reference drives multiple adcs, Typical qam demodulation application – Rainbow Electronics MAX1195 User Manual

Page 17

MAX1195

Dual, 8-Bit, 40Msps, 3V, Low-Power ADC with

Internal Reference and Parallel Outputs

______________________________________________________________________________________

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. The 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

P-P

. 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
modulation (QAM). Typically found in spread-spec-
trum-based systems, a QAM signal represents a carrier
frequency modulated in both amplitude and phase. At
the transmitter, modulating the baseband signal with

MAX4250

MAX6062

16.2k

Ω

162

Ω

3.3V

10Hz LOWPASS
FILTER

REFOUT

REFP

REFIN

µF

MAX1195

N = 1

REFN

N.C.

2.048V