DAC7741YCSLASH2K datasheet(16/23 Pages) TI1

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DAC7741

SBAS248B

www.ti.com

FIGURE 8. Offset Adjustment Transfer Characteristic.

OFFSET ADJUSTMENT

Offset adjustment is accomplished by introducing a small

current into the summing junction (SJ) of the DAC7741. The

voltage at SJ, or VSJ, is dependent on the output configura-

tion of the DAC7741. See Table IV for the required pin

strapping for a given configuration and the nominal values of

VSJ for each output range.

REFERENCE

OUTPUT

PIN STRAPPING

VSJ(1)

CONFIGURATION CONFIGURATION ROFFSET

RFB1

RFB2

Internal

0V to +10V

to VREF to VOUT to VOUT

+5V

Reference

–10V to +10V

to VOUT +3.333V

–5V to +5V

to AGND to VOUT to VOUT +1.666V

External

0V to VREF

to VREF to VOUT to VOUT

VREF/2

Reference

–VREF to VREF

to VOUT

VREF/3

–VREF/2 to VREF/2 to AGND to VOUT to VOUT

VREF/6

NOTE: (1) Voltage measured at VSJ for a given configuration.

TABLE IV. Nominal VSJ vs. VOUT and Reference Configuration.

OUTPUT

RPOT2

ISJ

NOMINAL

CONFIGURATION

RANGE

OFFSET

ADJUSTMENT

0V to +10V

10K

2.5M

±2µA

±25mV

–10V to +10V

10K

1.5M

±2.2µA

±55mV

–5V to +5V

10K

20K

±1.7µA

±21mV

TABLE V. Recommended External Component Values for

Symmetrical Offset Adjustment (VREF = 10V).

The current level required to adjust the DAC7741 offset can

be created by using a potentiometer divider as shown in

Figure 7. Another alternative is to use a unipolar DAC in

order to apply a voltage, VOADJ, to the resistor RS. A ±2uA

current range applied to SJ will ensure offset adjustment

coverage of the

±0.1% maximum offset specification of the

DAC7741.

When in a unipolar configuration (VSJ = 5V), only a single

resistor, RS, is needed for symmetrical offset adjustment with

a 0V to 10V VOADJ range. When in one of the two bipolar

configurations, VSJ is either +3.333v (±10V range) or +1.666V

(

±5V range), and circuit values chosen to match those given

in Table V will provide symmetrical offset adjust. Please refer

to Figure 7 for component configuration.

OFFSET ADJUST RANGE

–10V to +10V V

OUT

Configuration

min (75% of typ)

typ

–25

–50

–22

–11

SJ (µA)

0V to 10V and –5V to +5V

OUT Configuration

Figure 8 illustrates the typical and minimum offset adjustment

ranges provided by forcing a current at SJ for a given output

voltage configuration.

GAIN ADJUSTMENT

When using the internal reference of the DAC7741, gain

adjustment is performed by adjusting the internal refer-

ence voltage via the reference adjust pin, REFADJ. The

effect of a reference voltage change on the gain of the

DAC output can be seen in the generic equation (for

unipolar configuration):

VOUT = VREFIN • (N/65536)

where N is represented in decimal format and ranges from 0

to 65535.

REFADJ can be driven by a low impedance voltage source

such as a unipolar, 0V to +10V DAC or a potentiometer (less

than 100k

Ω) as shown in Figure 7. Since the input imped-

ance of REFADJ is typically 50k

Ω, the smaller the resistance

of the potentiometer, the more linear the adjustment will be.

A 10k

Ω potentiometer is suggested if linearity of the refer-

ence adjustment is of concern.

REF

OUT ADJUST RANGE

–10

–20

–30

–40

REFADJ (V)

Typical REF

OUT

Adjustment Range

Minimum REF

OUT

Adjustment Range

FIGURE 9. Internal Reference Adjustment Transfer Charac-

teristic.

VOLTAGE AT REFADJ

REFOUT VOLTAGE

REFADJ = 0V

10V + 25mV (min)

REFADJ = 5V or NC(1)

10V

REFADJ = 10V

10V – 25mV (max)

NOTE: "NC" is "Not Connected"

TABLE VI. Minimum Internal Reference Adjustment Range.

When the DAC7741 internal reference is not used, gain

adjustments can be made via trimming the external refer-

ence applied to the DAC at REFIN. This can be accomplished

through using a potentiometer, unipolar DAC, or other means

of precision voltage adjustment to control the voltage pre-

sented to the DAC7741 by the external reference. Figure 9

and Table VI summarize the range of adjustment of the

internal reference via REFADJ.

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