DAC7718 datasheet(39/60 Pages) TI

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= V

(Gain

OUT

REF

OFFSETDAC_CODE

4096

DAC7718

www.ti.com

SBAS467A – MAY 2009 – REVISED DECEMBER 2009

OFFSET DACS

There are two 12-bit Offset DACs: one for Group A, and one for Group B. The Offset DACs allow the entire

output curve of the associated DAC groups to be shifted by introducing a programmable offset. This offset allows

for asymmetric bipolar operation of the DACs or unipolar operation with bipolar supplies. Thus, subject to the

limitations of headroom, it is possible to set the output range of Group A and/or Group B to be unipolar positive,

unipolar negative, symmetrical bipolar, or asymmetrical bipolar, as shown in Table 5 and Table 6. Increasing the

digital input codes for the offset DAC shifts the outputs of the associated channels in the negative direction. The

default codes for the Offset DACs in the DAC7718 are factory trimmed to provide optimal offset and gain

performance for the default output range and span of symmetric bipolar operation. When the output range is

adjusted by changing the value of the Offset DAC, an extra offset is introduced as a result of the linearity and

offset errors of the Offset DAC. Therefore, the actual shift in the output span may vary slightly from the ideal

calculations. For optimal offset and gain performance in the default symmetric bipolar operation, the Offset DAC

input codes should not be changed from the default power-on values. The maximum allowable offset depends on

the reference and the power supply. If INPUT_CODE from Equation 1 or DAC_DATA_CODE from Equation 2 is

set to 0, then these equations simplify to Equation 3:

(3)

This equation shows the transfer function of the Offset DAC to the output of the DAC channels. In any case, the

analog output must not go beyond the specified range shown in the Analog Outputs section. After power-on or

reset, the Offset DAC is set to the value defined by the selected data format and the selected analog output

voltage. If the DAC gain setting is changed, the offset DAC code is reset to the default value corresponding to

the new DAC gain setting. Refer to the Power-On Reset and Hardware Reset sections for details.

For single-supply operation (AVSS = 0V), the Offset DAC is turned off, and the output amplifier is in a Hi-Z state.

The OFFSET-x pin must be connected to the AGND-x pin through a low-impedance connection (see the Layout

section for details). For dual-supply operation, this pin provides the output of the Offset DAC. The OFFSET-x pin

is not intended to drive an external load. See Figure 98 for the internal Offset DAC and output amplifier

configuration.

Table 5. Example of Offset DAC Codes and Output Ranges with Gain = 6 and VREF = 5V

OFFSET DAC

DAC CHANNELS MFS(1)

DAC CHANNELS PFS(1)

CODE

VOLTAGE

99Ah(2)

3.0V

–15V

+15V – 1 LSB

000h

+30V – 1 LSB

FFFh

~5.0V

–25V

+5V – 1 LSB

666h

~2.0V

–10V

+20V – 1 LSB

CCDh

~4.0V

–20V

+10V – 1 LSB

(1)

MFS = minus full-scale; PFS = plus full-scale.

(2)

This is the default code for symmetric bipolar operation; actual codes may vary ±1 LSB. Codes are in straight binary format.

Table 6. Example of Offset DAC Codes and Output Ranges with Gain = 4 and VREF = 5V

OFFSET DAC

DAC CHANNELS MFS(1)

DAC CHANNELS PFS(1)

CODE

VOLTAGE

AABh(2)

~3.33333V

–10V

+10V – 1 LSB

000h

+20V – 1 LSB

FFFh

~5.0V

–15V

+5V – 1 LSB

555h

~1.666V

–5V

+15V – 1 LSB

800h

2.5V

–7.5V

+12.5V – 1 LSB

D55h

~4.1666V

–12.5V

+7.5V – 1 LSB

(1)

MFS = minus full-scale; PFS = plus full-scale.

(2)

This is the default code for symmetric bipolar operation; actual codes may vary ±1 LSB. Codes are in straight binary format.

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