AD5347 datasheet(16/24 Pages) AD

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AD5346/AD5347/AD5348

Rev. 0 | Page 16 of 24

FUNCTIONAL DESCRIPTION

The AD5346/AD5347/AD5348 are octal resistor-string DACs

fabricated by a CMOS process with resolutions of 8, 10, and 12

bits, respectively. They are written to using a parallel interface.

They operate from single supplies of 2.5 V to 5.5 V, and the

output buffer amplifiers offer rail-to-rail output swing. The gain

of the buffer amplifiers can be set to 1 or 2 to give an output

voltage range of 0 V to VREF or 0 V to 2 × VREF. The AD5346/

AD5347/AD5348 have reference inputs that may be buffered to

draw virtually no current from the reference source. The devices

have a power-down feature that reduces current consumption

to only 100 nA @ 3 V.

DIGITAL-TO-ANALOG SECTION

The architecture of one DAC channel consists of a reference

buffer and a resistor-string DAC followed by an output buffer

amplifier. The voltage at the VREF pin provides the reference

voltage for the DAC. Figure 37 shows a block diagram of the

DAC architecture. Because the input coding to the DAC is

straight binary, the ideal output voltage is given by

Gain

REF

OUT

where:

is the decimal equivalent of the binary code, which is loaded

to the DAC register:

0–255 for AD5346 (8 bits)

0–1023 for AD5347 (10 bits)

0–4095 for AD5348 (12 bits)

is the DAC resolution.

Gain

is the output amplifier gain (1 or 2).

VOUTA

(GAIN = +1 OR +2)

VREFAB

BUF

DAC

INPUT

RESISTOR

STRING

OUTPUT

BUFFER AMPLIFIER

REFERENCE

BUFFER

Figure 37. Single DAC Channel Architecture

RESISTOR STRING

The resistor string section is shown in Figure 38. It is simply a

string of resistors, each of value R. The digital code loaded to

the DAC register determines at what node on the string the

voltage is tapped off to be fed into the output amplifier. The

voltage is tapped off by closing one of the switches connecting

the string to the amplifier. Because it is a string of resistors, it is

guaranteed monotonic.

TO OUTPUT

AMPLIFIER

VREF

Figure 38. Resistor String

DAC REFERENCE INPUT

The DACs operate with an external reference. The AD5346/

AD5347/AD5348 have a reference input for each pair of DACs.

The reference inputs may be configured as buffered or

unbuffered. This option is controlled by the BUF pin.

In buffered mode (BUF = 1), the current drawn from an

external reference voltage is virtually zero because the imped-

ance is at least 10 MΩ. The reference input range is 1 V to VDD.

In unbuffered mode (BUF = 0), the user can have a reference

voltage as low as 0.25 V and as high as VDD because there is no

restriction due to headroom and footroom of the reference

amplifier. The impedance is still large at typically 90 kΩ for 0 V

to VREF mode and 45 kΩ for 0 V to 2 × VREF mode.

If using an external buffered reference (such as REF192), there

is no need to use the on-chip buffer.

OUTPUT AMPLIFIER

The output buffer amplifier is capable of generating output

voltages to within 1 mV of either rail. Its actual range depends

on VREF, GAIN, the load on VOUT, and offset error.

If a gain of +1 is selected (GAIN = 0), the output range is

0.001 V to VREF.

If a gain of +2 is selected (GAIN = +1), the output range is

0.001 V to 2 × VREF. However, because of clamping, the

maximum output is limited to VDD – 0.001 V.

The output amplifier is capable of driving a load of 2 kΩ to

GND or VDD, in parallel with 500 pF to GND or VDD. The source

and sink capabilities of the output amplifier can be seen in

Figure 24.

The slew rate is 0.7 V/µs with a half-scale settling time to ±0.5 LSB

(at 8 bits) of 6 s with the output unloaded. See Figure 29.

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