AD7242(RevB) 查看數據表（PDF） - Analog Devices

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AD7242
(Rev.:RevB)

LC2MOS Dual, Complete, 12-Bit/14-Bit Serial DACs

Analog Devices 

AD7242

CIRCUIT DESCRIPTION

The AD7242/AD7244 contains two 12-bit/14-bit D/A convert-

ers, each with an output buffer amplifier. The part also contains

a reference input buffer amplifier for each DAC, and an on-chip

3 V reference.

D/A Section

The AD7242/AD7244 contains two 12-bit/14-bit voltage mode

D/A converters, each consisting of highly stable thin-film resistors

and high speed single-pole, double-throw switches. The simplified

circuit diagram for the DAC section is shown in Figure 1. The

three MSBs of the data word are decoded to drive the seven

switches A-G. On the AD7242, the 9 LSBs switch a

9-bit R-2R ladder structure while on the AD7244, the 11 LSBs

switch an 11-bit R-2R ladder structure. The output voltage

from this converter has the same polarity as the reference

voltage, REF IN.

The REF IN voltage is internally buffered by a unity gain

amplifier before being applied to the D/A converters and the

bipolar bias circuitry. The D/A converter is configured and

scaled for a 3 V reference, and the device is tested with 3 V

applied to REF IN. Operating the AD7242/AD7244 at refer-

ence voltages outside the ± 5% tolerance range may result in

degraded performance from the part.

ETE Figure 1. DAC Ladder Structure

L Internal Reference

The on-chip reference is a temperature-compensated buried

Zener reference that is factory trimmed for 3 V ± 10 mV. The

O reference can be used to provide both the reference voltage for

the two D/A converters and the bipolar biasing circuitry. This is

achieved by connecting REF OUT to REF INA and REF INB.

The reference voltage can also be used for other components

S and is capable of providing up to 500 μA to an external load.

The maximum recommended capacitance on the reference

output pin for normal operation is 50 pF. If the reference

output is required to drive a capacitive load greater than 50 pF,

B a 200 Ω resistor should be placed in series with the capacitive

load. Decoupling the REF OUT pin with a series 200 Ω resistor

and a parallel combination of a 10 μF tantalum capacitor and a

0.1 μF ceramic capacitor as in Figure 2 reduces the noise

O spectral density of the reference (see Figure 4). Using this

Figure 2. Circuit Connection for REF OUT with an External

Capacitive Load of Greater Than 50 pF

decoupling scheme to generate the reference voltage for REF

INA and REF INB gives a channel-to-channel isolation number

of 110 dB (connecting REF OUT directly to REF INA and

REF INB gives 80 dB). The channel-to-channel isolation is 110

dB using an external reference.

External Reference

In some applications, the user may require a system reference or

some other external reference to drive the AD7242/AD7244

reference inputs. Figure 3 shows how the AD586 reference can

be conditioned to provide the 3 V reference required by the

AD7242/AD7244 reference inputs.

Figure 3. AD586 Driving AD7242/AD7244 Reference Inputs

–6–

REV. B

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