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AD5381

Rev. A | Page 21 of 36

FUNCTIONAL DESCRIPTION

DAC ARCHITECTURE—GENERAL

The AD5381 is a complete, single-supply, 40-channel voltage

output DAC that offers 12-bit resolution. The part is available in

a 100-lead LQFP package and features both a parallel and a

serial interface. This product includes an internal, software

selectable, 1.25 V/2.5 V, 10 ppm/°C reference that can be used to

drive the buffered reference inputs; alternatively, an external

reference can be used to drive these inputs. Internal/external

reference selection is via the CR8 bit in the control register;

CR10 selects the reference magnitude if the internal reference is

selected. All channels have an on-chip output amplifier with

rail-to-rail output capable of driving 5 kΩ in parallel with a

200 pF load.

VOUT

12-BIT

DAC

REG

m REG

c REG

×1 INPUT

REG

×2

INPUT DATA

VREF

AVDD

Figure 27. Single-Channel Architecture

The architecture of a single DAC channel consists of a 12-bit

resistor-string DAC followed by an output buffer amplifier

operating at a gain of 2. This resistor-string architecture

guarantees DAC monotonicity. The 12-bit binary digital code

loaded to the DAC register determines at what node on the

string the voltage is tapped off before being fed to the output

amplifier. Each channel on these devices contains independent

offset and gain control registers that allow the user to digitally

trim offset and gain. These registers give the user the ability to

calibrate out errors in the complete signal chain, including the

DAC, using the internal m and c registers, which hold the

correction factors. All channels are double buffered, allowing

synchronous updating of all channels using the LDAC pin.

Figure 27 shows a block diagram of a single channel on the

AD5381. The digital input transfer function for each DAC can

be represented as

x2 = [(m + 2)/ 2n × x1] + (c – 2n – 1)

where:

x2 = the data-word loaded to the resistor string DAC

x1 = the 12-bit data-word written to the DAC input register

m = the gain coefficient (default is 0xFFE). The gain coefficient

is written to the 11 most significant bits (DB11–DB1), the LSB

(DB0) of the data-word is a 0.

n = DAC resolution (n = 12 for AD5381)

c is the12-bit offset coefficient (default is 0x800).

The complete transfer function for these devices can be

represented as

VOUT = 2 × VREF × x2/2n

where:

x2 is the data-word loaded to the resistor string DAC. VREF is the

internal reference voltage or the reference voltage externally

applied to the DAC REFOUT/REFIN pin. For specified

performance, an external reference voltage of 2.5 V is

recommended for the AD5381-5, and 1.25 V for the AD5381-3.

Data Decoding

The AD5381 contains a 12-bit data bus, DB11–DB0. Depending

on the value of REG1 and REG0 (see Table 11), this data is

loaded into the addressed DAC input registers, offset (c)

registers, or gain (m) registers. The format data, offset (c), and

gain (m) register contents are shown in Table 12 to Table 14.

Table 11. Register Selection

REG1

REG0

Input Data Register (x1)

Offset Register (c)

Gain Register (m)

Special Function Registers (SFRs)

Table 12. DAC Data Format (REG1 = 1, REG0 = 1)

DB11 to DB0

DAC Output (V)

1111

2 VREF × (4095/4096)

1111

1110

2 VREF × (4094/4096)

1000

0000

0001

2 VREF × (2049/4096)

1000

0000

2 VREF × (2048/4096)

0111

1111

2 VREF × (2047/4096)

0000

0001

2 VREF × (1/4096)

0000

Table 13. Offset Data Format (REG1 = 1, REG0 = 0)

DB11 to DB0

Offset (LSB)

1111

+2048

1111

1110

+2047

1000

0000

0001

1000

0000

0111

1111

–1

0000

0001

–2047

0000

–2048

Table 14. Gain Data Format (REG1 = 0, REG0 = 1)

DB11 to DB0

Gain Factor

1111

1110

1011

1111

1110

0.75

0111

1111

1110

0.5

0011

1111

1110

0.25

0000

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