5962-9756401QXA Datasheet, PDF (11/16 Pages) Analog Devices – 16-Bit, 100 kSPS/200 kSPS BiCMOS A/D Converters

AD976/AD976A
VOLTAGE REFERENCE
The AD976/AD976A has an on-chip temperature compensated
bandgap voltage reference that is factory trimmed to 2.5 V
± 20 mV. The full-scale range of the ADC is equal to ± 4 V_REF.
Thus, the nominal range will be ± 10 V.
The accuracy of the AD976 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
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 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 im-
pedance at this point is typically 1 ohm. Because of this 1 ohm
output impedance, it is important to minimize any ac or input
dependent loads that will lead to increased distortion. Any dc
loads will simply act as a gain error. Although the typical char-
acteristic of Figure 14 shows that the AD976 is capable of driv-
ing loads greater than 15 mA, it is not recommended that the
steady state current exceed 2 mA.
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 9 shows the AD976/AD976A 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 REF192 provides
a low quiescent current, high accuracy and low temperature
drift solution.
0.1␮F
+5V
؎10V INPUT
TEMP V_OUT
AD780
V_IN
GND
R1
200⍀
R2
33.2k⍀
C1
2.2␮F
C3
C4
1␮F
0.1␮F
C2
2.2␮F
V_IN
REF _AD976/
AD976A
AGND1
V_ANA
CAP
AGND2
Figure 9. AD780 External Reference Connection to the
AD976/AD976A
AC PERFORMANCE
The AD976/AD976A is fully specified and tested for dynamic
performance specifications. The ac parameters are required for
signal processing applications such as speech recognition and
spectrum analysis. These applications require information on
the ADC’s effect on the spectral content of the input signal.
Hence, the parameters for which the AD976/AD976A is
specified include: S/(N+D), THD and Spurious Free Dynamic
Range. These terms are discussed in greater detail in the follow-
ing sections.
As a general rule, it is recommended that the results from sev-
eral conversions be averaged to reduce the effects of noise, thus
improving parameters such as S/(N+D) and THD. The ac per-
formance of the AD976/AD976A can be optimized by operating
the ADC at its maximum sampling rate of 100 kHz/200 kHz
and by digitally filtering the resulting bit stream to the desired
signal bandwidth. By distributing noise over a wider frequency
range, the noise density in the frequency band of interest can be
reduced. For example, if the required input bandwidth is 50 kHz,
the AD976A could be oversampled by a factor of 2. This would
yield a 3 dB improvement in the effective SNR performance.
100%
0
–10
–20
F_SAMPLE = 200kHz
–30
F_IN = 45kHz
SNR = 86.23dB
–40
THD = –105.33dB
–50
–60
–70
–80
–90
–100
–110
–120
–130
–140
–150
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95100
FREQUENCY – kHz
Figure 10. FFT PLOT
DC PERFORMANCE
The factory calibration scheme used for the AD976/AD976A
compensates for bit weight errors that may exist in the capacitor
array. The mismatch in capacitor values is adjusted (using the
calibration coefficients) during a conversion, resulting in excellent
dc linearity performance. Figures 11, 12, 15, 16, 17 and 18,
respectively, show typical INL, typical DNL, typical positive and
negative INL and DNL distribution plots for the AD976/AD976A
at +25°C.
A histogram test is a statistical method for deriving an A/D
converter’s differential nonlinearity. A ramp input is sampled
by the ADC and a large number of conversions are taken and
stored. Theoretically, the codes would all be the same size and
therefore have an equal number of occurrences. A code with an
average number of occurrences would have a DNL of “0.” A
code that is different than the average would have a DNL that
was either greater or less than zero LSB. A DNL of –1 LSB
indicates that there is a missing code present at the 16-bit level
and that the ADC exhibits 15-bit performance.
REV. C
–11–