AD7228A Просмотр технического описания (PDF) - Analog Devices

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AD7228A

LC2MOS Octal 8-Bit DAC

Analog Devices 

AD7228A

OP AMP SECTION

Each voltage-mode D/A converter output is buffered by a unity

gain noninverting CMOS amplifier. This buffer amplifier is

tested with a 2 kΩ and 100 pF load but will typically drive a

2 kΩ and 500 pF load.

The AD7228A can be operated single or dual supply. Operating

the part from single or dual supplies has no effect on the positive-

going settling time. However, the negative-going settling time to

voltages near 0 V in single supply will be slightly longer than the

settling time for dual supply operation. Additionally, to ensure

that the output voltage can go to 0 V in single supply, a transis-

tor on the output acts as a passive pull-down as the output volt-

age nears 0 V. As a result, the sink capability of the amplifier is

reduced as the output voltage nears 0 V in single supply. In dual

supply operation, the full sink capability of 400 µA at 25°C is

maintained over the entire output voltage range. The single sup-

ply output sink capability is shown in Figure 4. The negative

VSS also gives improved output amplifier performance allowing

an extended input reference voltage range and giving improved

slew rate at the output.

SUPPLY CURRENT

The AD7228A has a maximum IDD specification of 22 mA and

a maximum ISS of 20 mA over the –55°C to +125°C tempera-

ture range. This maximum current specification is actually de-

termined by the current at –55°C. Figure 6 shows a typical plot

of power supply current versus temperature.

Figure 4. Single Supply Sink Current

The output broadband noise from the amplifier is 300 µV

peak-to-peak. Figure 5 shows a plot of noise spectral density

versus frequency.

Figure 6. Power Supply Current vs. Temperature

APPLYING THIS AD7228A

UNIPOLAR OUTPUT OPERATION

This is the basic mode of operation for each channel of the

AD7228A, with the output voltage having the same positive po-

larity as VREF. Connections for unipolar output operation are

shown in Figure 7. The AD7228A can be operated from single

or dual supplies as outlined earlier. The voltage at the reference

input must never be negative with respect to GND. Failure to

observe this precaution may cause parasitic transistor action and

possible device destruction. The code table for unipolar output

operation is shown in Table II.

Figure 5. Noise Spectral Density vs. Frequency

DIGITAL INPUTS

The AD7228A digital inputs are compatible with either TTL or

5 V CMOS levels. All logic inputs are static-protected MOS

gates with typical input currents of less than 1 nA. Internal in-

put protection is achieved by on-chip distributed diodes.

REV. A

–5–

Figure 7. Unipolar Output Circuit

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