Rainbow Electronics MAX5888 User Manual

Page 12

MAX5888

3.3V, 16-Bit, 500Msps High Dynamic
Performance DAC with Differential LVDS Inputs

______________________________________________________________________________________

amplitudes. It is also recommended to center tap the
transformer to ground. If no transformer is used, each
DAC output should be terminated to ground with a 50

Ω

resistor. Additionally, a 100

Ω 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 MAX5888 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 MAX5888 is optimized for a 50

Ω

double termination. It can be used with a transformer
output as shown in Figure 7 or just one 50

Ω resistor

from each output to ground and one 50

Ω 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 section), a transmit mask (Tx mask)
exists for this application. The spread-spectrum modula-
tion function applied to the carrier frequency generates a
spectral response, which is uniform over a given band-
width (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
over the adjacent sideband. Four contiguous WCDMA

MAX5888

T2, 1:1

T1, 1:1

OUT

, SINGLE ENDED

WIDEBAND RF TRANSFORMER T2

PERFORMS THE DIFFERENTIAL TO

SINGLE-ENDED CONVERSION.

Ω

100

Ω

IOUTP

IOUTN

B0–B15

VCLK

AGND

DGND

CLKGND

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