Unbuffered external reference drives multiple adcs, Typical qam demodulation application – Rainbow Electronics MAX1198 User Manual

Page 17

MAX1198

Dual, 8-Bit, 100Msps, 3.3V, Low-Power ADC

with Internal Reference and Parallel Outputs

______________________________________________________________________________________

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 indi-
vidual voltage followers are connected to 10Hz low-
pass 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
quadrature outputs, a local oscillator followed by sub-
sequent upconversion can generate the QAM signal.
The result is an in-phase (I) and a quadrature (Q) carri-
er component, where the Q component is 90° phase

MAX4250

MAX6062

16.2k

Ω

162

Ω

3.3V

10Hz LOWPASS
FILTER

REFOUT

REFP

REFIN

µF

MAX1198

N = 1

REFN

N.C.

2.048V