Rainbow Electronics MAX5887 User Manual

Page 12

MAX5887

3.3V, 14-Bit, 500Msps High Dynamic
Performance DAC with Differential LVDS Inputs

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transformer to ground. If no transformer is used, each
DAC output should be terminated to ground with a 50Ω
resistor. Additionally, a 100Ω resistor should be placed
between the outputs (Figure 8).

If a single-ended unipolar output is desirable, IOUTP
should be selected as the output, with IOUTN ground-
ed. However, driving the MAX5887 single ended is not
recommended since additional noise is added (from
the ground plane) in such configurations.

The distortion performance of the DAC depends on the
load impedance. The MAX5887 is optimized for a 50Ω
double termination. It can be used with a transformer
output as shown in Figure 7 or just one 50Ω resistor
from each output to ground and one 50Ω resistor
between the outputs. This produces a full-scale output
power of up to 0dBm depending on the output current
setting. Higher termination impedance can be used at
the cost of degraded distortion performance and
increased output noise voltage.

Adjacent Channel Leakage Power Ratio

(ACLR) Testing for CDMA- and

WCDMA-Based Base Station

Transceiver Systems (BTS)

The transmitter sections of BTS applications serving
CDMA and WCDMA architectures must generate carri-
ers with minimal coupling of carrier energy into the adja-
cent channels. Similar to the GSM/EDGE model (see the
Multitone Testing for GSM/EDGE Applications section in
the Applications Information section), a transmit mask
(Tx mask) exists for this application. The spread-spec-
trum modulation function applied to the carrier frequency
generates a spectral response, which is uniform over a
given bandwidth (up to 4MHz) for a WCDMA-modulated
carrier.

A dominant specification is ACLR, a parameter which
reflects the ratio of the power in the desired carrier
band to the power in an adjacent carrier band. The
specification covers the first two adjacent bands, and is
measured on both sides of the desired carrier.

According to the transmit mask for CDMA and WCDMA
architectures, the power ratio of the integrated carrier
channel energy to the integrated adjacent channel
energy must be >45dB for the first adjacent carrier slot
(ACLR 1) and >50dB for the second adjacent carrier
slot (ACLR 2). This specification applies to the output of
the entire transmitter signal chain. The requirement for
only the DAC block of the transmitter must be tighter,
with a typical margin of >15dB, requiring the DAC’s
ACLR 1 to be better than 60dB. Adjacent channel leak-
age is caused by a single-spread spectrum carrier,
which generates intermodulation (IM) products
between the frequency components located within the
carrier band. The energy at one end of the carrier band
generates IM products with the energy from the oppo-
site end of the carrier band. For single-carrier WCDMA
modulation, these IMD products are spread 3.84MHz

MAX5887

T2, 1:1

T1, 1:1

OUT

, SINGLE ENDED

WIDEBAND RF TRANSFORMER T2

PERFORMS THE DIFFERENTIAL TO

SINGLE-ENDED CONVERSION.

50Ω

100Ω

50Ω

IOUTP

IOUTN

B0–B13

VCLK

AGND

DGND

CLKGND

Figure 7. Differential to Single-Ended Conversion Using a Wideband RF Transformer