AD977ABR datasheet(18/24 Pages) AD

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AD977/AD977A

–18–

REV. D

VOLTAGE REFERENCE

The AD977/AD977A has an on-chip temperature compensated

bandgap voltage reference that is factory trimmed to 2.5 V

±20 mV. The accuracy of the AD977/AD977A over the speci-

fied temperature ranges is dominated by the drift performance

of the voltage reference. 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 14, which is a plot of the

change in reference voltage (in mV) versus the change in tem-

perature—notice the plot is normalized for zero error at 25

°C.

If improved drift performance 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 onboard reference

buffer is provided. The output of this buffer is provided at the

CAP pin and is available to the user; however, when externally

loading the reference buffer, it is important to make sure that

proper precautions are taken to minimize any degradation in the

ADC’s performance. Figure 15 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 impedance at this point is typically 1

Ω. Because of this

Ω output impedance, it is important to minimize any ac or

DEGREES CELSIUS

–55

125

Figure 14. Reference Drift

LOAD CURRENT – 5mA/DIV

SOURCE CAPABILITY

SINK CAPABILITY

Figure 15. CAP Pin Load Regulation

input dependent loads that will lead to increased distortion.

Any dc loads will simply act as a gain error. Although the typi-

cal characteristic of Figure 15 shows that the AD977/AD977A

is capable of driving loads greater than 15 mA, it is recom-

mended that the steady state current not exceed 2 mA.

Using an External Reference

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

ence 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.

Figures 16a and 16b show the AD977/AD977A used in bipolar

mode with the AD780 voltage reference applied to the REF

pin. It is important to note that in Figure 16a the R3IN pin is

connected to the CAP pin whereas in Figure 16b the R3IN pin

of the AD977A is returned to the output of the external refer-

ence. The AD780 is a bandgap reference that exhibits ultralow

drift, low initial error and low output noise. In Figure 16b,

the value for C1 is only applicable to applications using the

AD780. In applications using a different external reference a

different value for C1 may be required. For low power applica-

tions, the REF192 provides a low quiescent current, high

accuracy and low temperature drift solution.

200

VIN

33.2k

2.2 F

R1IN

R2IN

R3IN

REF

AGND2

CAP

AGND1

AD977

VANA

0.1 F

2.2 F

1 F

GND

VOUT

VIN

TEMP

AD780

0.1 F

100

Figure 16a. AD780 External Reference to AD977 Config-

ured for

±10 V Input Range

200

VIN

33.2k

2.2 F

R1IN

R2IN

R3IN

REF

AGND2

CAP

AGND1

AD977A

VANA

0.1 F

330 F*

1 F

GND

VOUT

VIN

TEMP

AD780

0.1 F

100

*ESR AT 100kHz MUST BE LESS THAN 0.3 .

RECOMMEND KEMET T495 SERIES OR

SANYO 6SA330M.

Figure 16b. AD780 External Reference to AD977A Config-

ured for

±10 V Input Range

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