MAX5877EGK+D【PDF 7/19 页】IC DAC 14BIT DUAL 250MSPS 68-QFN

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MAX5877

14-Bit, 250Msps, High-Dynamic-Performance,

Dual DAC with LVDS Inputs

______________________________________________________________________________________

Differential-to-Single-Ended Conversion

Using a Wideband RF Transformer

Use a pair of transformers (Figure 7) or a differential

amplifier configuration to convert the differential voltage

existing between OUTIP/OUTQP and OUTIN/OUTQN to

a single-ended voltage. Optimize the dynamic perfor-

mance by using a differential transformer-coupled out-

put and limit the output power to < 0dBm full scale. Pay

close attention to the transformer core saturation char-

acteristics when selecting a transformer for the

MAX5877. Transformer core saturation can introduce

strong 2nd-order harmonic distortion especially at low

output frequencies and high signal amplitudes. For best

results, center tap the transformer to ground. When not

using a transformer, terminate each DAC output to

ground with a 25

resistor. Additionally, place a 50

resistor between the outputs (Figure 8).

For a single-ended unipolar output, select OUTIP

(OUTQP) as the output and ground OUTIN (OUTQN).

Driving the MAX5877 single-ended is not recommend-

ed since additional noise and distortion will be added.

The distortion performance of the DAC depends on the

load impedance. The MAX5877 is optimized for 50

differential double termination. It can be used with a

transformer output as shown in Figure 7 or just one 25

resistor from each output to ground and one 50

resis-

tor between the outputs (Figure 8). This produces a full-

scale output power of up to -2dBm, depending on the

output current setting. Higher termination impedance

can be used at the cost of degraded distortion perfor-

mance and increased output noise voltage.

Grounding, Bypassing, and Power-

Supply Considerations

Grounding and power-supply decoupling can strongly

influence the MAX5877 performance. Unwanted digital

crosstalk couples through the input, reference, power

supply, and ground connections, and affects dynamic

performance. High-speed, high-frequency applications

require closely followed proper grounding and power-

supply decoupling. These techniques reduce EMI and

internal crosstalk that can significantly affect the

MAX5877 dynamic performance.

Use a multilayer PCB with separate ground and power-

supply planes. Run high-speed signals on lines directly

above the ground plane. Keep digital signals as far away

from sensitive analog inputs and outputs, reference input

sense lines, and clock inputs as practical. Use a con-

trolled-impedance, symmetric, differential design of data

input, clock input, and the analog output lines to minimize

2nd-order harmonic distortion and noise components,

thus optimizing the DAC’s dynamic performance. Keep

digital signal paths short and run lengths matched to

avoid propagation delay and data skew mismatches.

The MAX5877 requires five separate power-supply inputs

for analog (AVDD1.8 and AVDD3.3), digital (DVDD1.8 and

DVDD3.3), and clock (AVCLK) circuitry. All power-supply

pins must be connected to their proper supply. Decouple

each AVDD, DVDD, and AVCLK input pin with a separate

0.1F capacitor as close to the device as possible with

the shortest possible connection to the ground plane

(Figure 9). Minimize the analog and digital load capaci-

tances for optimized operation. Decouple all three

power-supply voltages at the point they enter the PCB

with tantalum or electrolytic capacitors. Ferrite beads with

additional decoupling capacitors forming a pi-network

could also improve performance.

MAX5877

OUTIP/OUTQP

OUTIN/OUTQN

DATA13–DATA0

WIDEBAND RF TRANSFORMER T2 PERFORMS THE

DIFFERENTIAL-TO-SINGLE-ENDED CONVERSION

T1, 1:1

T2, 1:1

GND

100

VOUT, SINGLE-ENDED

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

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