AD974 datasheet(15/20 Pages) AD

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REV. A

AD974

–15–

OFFSET AND GAIN ADJUSTMENT

The AD974 is factory trimmed to minimize gain, offset and

linearity errors. There are no internal provisions to allow for any

further adjustment of offset error through external circuitry.

The reference of the AD974 can be adjusted as shown in Figure

12. This will allow the full-scale error of any one channel to be

adjusted to zero or will allow the average full-scale error of the

four channels to be minimized.

2.2 F

576k

50k

CAP

REF

AGND2

AD974

+5V

Figure 12. AD974 Full-Scale Trim

VOLTAGE REFERENCE

The AD974 has an on-chip temperature compensated bandgap

voltage reference that is factory trimmed to +2.5 V

± 20 mV.

The accuracy of the AD974 over the specified temperature

range is dominated by the drift performance of the voltage refer-

ence. The on-chip voltage reference is laser-trimmed to provide

a typical drift of 7 ppm/

°C. This typical drift characteristic is

shown in Figure 13, which is a plot of the change in reference

voltage (in mV) versus the change in temperature—notice the

plot is normalized for zero error at +25

°C. If improved drift perfor-

mance is required, an external reference such as the AD780

should be used to provide a drift as low as 3 ppm/

°C. In order to

simplify the drive requirements of the voltage reference (internal

or external), an on-chip reference buffer is provided.

–55

125

DEGREES – Celsius

Figure 13. Reference Drift

The output of this buffer is provided at the CAP pin and is

available to the user; however, when externally loading the refer-

ence buffer, it is important to make sure that proper precautions

are taken to minimize any degradation in the ADC’s perfor-

mance. Figure 14 shows the load regulation of the reference

buffer. Notice that this figure is also normalized so that there is

zero error with no dc load. In the linear region, the output imped-

ance at this point is typically 1

Ω. Because of this output imped-

ance, it is important to minimize any ac- or input-dependent

loads that will lead to increased distortion. Any dc load will

simply act as a gain error. Although the typical characteristic of

Figure 14 shows that the AD974 is capable of driving loads

greater than 15 mA, it is recommended that the steady state

current not exceed 2 mA.

LOAD CURRENT – 5mA/DIV

SOURCE CAPABILITY

SINK CAPABILITY

Figure 14. CAP Pin Load Regulation

Using an External Reference

In addition to the on-chip reference, an external 2.5 V reference

can be applied. When choosing an external reference for a

16-bit application, however, careful attention should be paid to

noise and temperature drift. These critical specifications can

have a significant effect on the ADC performance.

Figure 15 shows the AD974 used in bipolar mode with the

AD780 voltage reference applied to the REF pin. The AD780

is a bandgap reference that exhibits ultralow drift, low initial

error and low output noise. For low power applications, the

AD780 provides a low quiescent current, high accuracy and low

temperature drift solution.

BIP

VxA

VxB

AGND1

CAP

REF

AGND2

AD974

VIN

0.1 F

2.2 F

VANA

TEMP

VOUT

GND

VIN

AD780

2.2 F

–

1 F

–

+5V

0.1 F

–

Figure 15. External Reference to AD974 Configured for

±10 V Input Range

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