THS5661A_15 Datasheet, PDF (16/34 Pages) Texas Instruments – DIGITAL-TO-ANALOG CONVERTER

THS5661A
12ĆBIT, 125 MSPS, CommsDAC
DIGITALĆTOĆANALOG CONVERTER
SLAS247B − NOVEMBER 1999 REVISED SEPTEMBER 2002
APPLICATION INFORMATION
The THS5661A architecture is based on current steering, combining high update rates with low power
consumption. The CMOS device consists of a segmented array of PMOS transistor current sources, which are
capable of delivering a full-scale current up to 20 mA. High-speed differential current switches direct the current
of each current source to either one of the output nodes, IOUT1 or IOUT2. The complementary output currents
thus enable differential operation, canceling out common mode noise sources (on-chip and PCB noise), dc
offsets, even order distortion components, and increasing signal output power by a factor of two. Major
advantages of the segmented architecture are minimum glitch energy, excellent DNL, and very good dynamic
performance. The DAC’s high output impedance of >300 kΩ and fast switching result in excellent dynamic
linearity (spurious free dynamic range SFDR).
The full-scale output current is set using an external resistor R_BIAS in combination with an on-chip bandgap
voltage reference source (1.2 V) and control amplifier. The current I_BIAS through resistor R_BIAS is mirrored
internally to provide a full-scale output current equal to 32 times I_BIAS. The full-scale current can be adjusted
from 20 mA down to 2 mA.
data interface and timing
The THS5661A comprises separate analog and digital supplies, i.e. AV_DD and DV_DD. The digital supply voltage
can be set from 5.5 V down to 3 V, thus enabling flexible interfacing with external logic. The THS5661A provides
two operating modes, as shown in Table 1. Mode 0 (mode pin connected to DGND) supports a straight binary
input data word format, whereas mode 1 (mode pin connected to DV_DD) sets a twos complement input
configuration.
Figure 29 shows the timing diagram. Internal edge-triggered flip-flops latch the input word on the rising edge
of the input clock. The THS5661A provides for minimum setup and hold times (> 1 ns), allowing for noncritical
external interface timing. Conversion latency is one clock cycle for both modes. The clock duty cycle can be
chosen arbitrarily under the timing constraints listed in the digital specifications table. However, a 50% duty cycle
will give optimum dynamic performance. Figure 30 shows a schematic of the equivalent digital inputs of the
THS5661A, valid for pins D11−D0, SLEEP, and CLK. The digital inputs are CMOS-compatible with logic
thresholds of DV_DD/2 ±20%. Since the THS5661A is capable of being updated up to 125 MSPS, the quality of
the clock and data input signals are important in achieving the optimum performance. The drivers of the digital
data interface circuitry should be specified to meet the minimum setup and hold times of the THS5661A, as well
as its required min/max input logic level thresholds. Typically, the selection of the slowest logic family that
satisfies the above conditions will result in the lowest data feed-through and noise. Additionally, operating the
THS5661A with reduced logic swings and a corresponding digital supply (DV_DD) will reduce data feed-through.
Note that the update rate is limited to 70 MSPS for a digital supply voltage DV_DD of 3 V to 3.6 V.
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