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
1 Ω output impedance, it is important to minimize any ac or
DEGREES CELSIUS
–55
1mV/DIV
25 125
Figure 14. Reference Drift
LOAD CURRENT – 5mA/DIV
dV ON CAP PIN – 10mV/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 R3
IN
pin is
connected to the CAP pin whereas in Figure 16b the R3
IN
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
V
IN
33.2k
C2
2.2F
R1
IN
R2
IN
R3
IN
REF
AGND2
CAP
AGND1
AD977
V
ANA
C4
0.1F
C1
2.2F
C3
1F
4
GND
6
V
OUT
V
IN
TEMP
AD780
3
2
0.1F
5V
100
Figure 16a. AD780 External Reference to AD977 Config-
ured for
±
10 V Input Range
200
V
IN
33.2k
C2
2.2F
R1
IN
R2
IN
R3
IN
REF
AGND2
CAP
AGND1
AD977A
V
ANA
C4
0.1F
C1
330F*
C3
1F
4
GND
6
V
OUT
V
IN
TEMP
AD780
3
2
0.1F
5V
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
