AD1876JN Analog Devices, AD1876JN Datasheet - Page 8

• Manufacturer Part Number: AD1876JN
• Description: 16-Bit 100 kSPS Sampling ADC
• Manufacturer: Analog Devices

AD1876
VOLTAGE REFERENCE
The AD1876 requires the use of an external voltage reference.
The input voltage range is determined by the value of the refer-
ence voltage; in general, a reference voltage of n volts produces
an input range of n volts. Signal-to-noise performance is in-
creased proportionately with input signal range. The AD1876 is
specified with a 5.0 V reference and an analog input of 5 V. In
the presence of a fixed amount of system noise, increasing the
LSB size (which results from increasing the reference voltage)
will increase the effective S/(N+D) performance for input values
below the point where input distortion occurs. Figure 11 illus-
trates S/(N+D) as a function of input amplitude and reference
voltage.
During a conversion, the switched capacitor array of the
AD1876 presents a dynamically changing current load at the
voltage reference as the successive-approximation algorithm
cycles through various choices of capacitor weighting. The out-
put impedance of the reference circuitry must be low so that the
output voltage will remain sufficiently constant as the current
drive changes. In most applications, this requires that the output
of the voltage reference be buffered by an amplifier with low im-
pedance at relatively high frequencies. A (10 F or larger) ca-
pacitor connected between V
demands on the reference by decreasing the magnitude of high
frequency components.
The following two sections represent typical design approaches.
VOLTAGE REFERENCE—AUDIO APPLICATIONS
Audio applications require optimal ac performance over a rela-
tively narrow temperature range, with low cost being important.
Figure 4 shows one such approach towards attaining these goals.
A voltage reference, consisting of a Zener diode, capacitor, resis-
tor and op amp with typical component values, is shown. This
simple circuit has the advantage of low cost, but the reference
voltage value is sensitive to changes in the +12 V supply. Addi-
tionally, changes in the Zener value due to temperature varia-
tions will also be reflected in the reference voltage. R
be required for other component selections if the Zener requires
more current than the op amp can supply.
VOLTAGE REFERENCE—PRECISION MEASUREMENT
APPLICATIONS
In applications other than audio, parameters such as low drift
over temperature and static accuracy are important. Figure 5
shows a voltage reference circuit featuring the 5 V AD586. The
AD586 is a low cost reference which utilizes a buried Zener ar-
chitecture to provide low noise and drift. Over the 0 C to
R
OPTION
Figure 4. Low Cost Voltage Reference Circuit
AD589
+
1k
0.1 F
REF
and AGND will reduce the
AD711
3k
1k
11
OPTION
AD1876
V
REF
may
–8–
+70 C range, the AD586L grade exhibits less than a 2.25 mV
output change from its initial value at +25 C. A noise-reduction
capacitor, C
put, thereby optimizing the overall performance of the AD1876.
For higher performance needs, the AD588 reference provides
improved drift, low noise, and excellent initial accuracy. The
AD588 uses a proprietary ion-implanted buried Zener diode in
conjunction with laser-trimmed thin-film resistors for low offset
and gain. The AD588 output is accurate to 0.65 mV from its
value at +25 C over the 0 C to +70 C range. The circuit shown
in Figure 6 includes a noise-reduction network on Pins 4, 6 and
7. The 1 F capacitors form low-pass filters with the internal re-
sistance of the AD588 and external 3.9 k resistor. This re-
duces the wide-band (to 1 MHz) noise of the AD588, providing
optimum performance of the AD1876.
ANALOG INPUT
As previously discussed, the analog input voltage range for the
AD1876 is V
mode rejection, the V
ground. V
(AGND), and V
(AGND SENSE) which allows a remote ground sense for the
input signal. If AGND SENSE is not used, it should be con-
nected to the AGND pin at the package. The AGND SENSE
pin is intended to be tied to potentials within 100 mV of AGND
to maintain specified performance.
The AD1876 analog inputs (V
exhibit dynamic characteristics. When a conversion cycle begins,
each analog input is connected to an internal, discharged 50 pF
capacitor which then charges to the voltage present at the
–12V
+12V
REDUCTION
0.1µF
0.1µF
REF
1 F
C
NOISE
N
8
N
, reduces the broadband noise of the AD586 out-
is referred to the local analog system ground
REF
AD586
1µF
16
IN
2
+12V
. For purposes of ground drop and common-
2
4
is referred to the analog ground sense pin
NC
7
5
IN
AD588
NC
and V
6
6
8
Figure 5.
3.9k
Figure 6.
13
4
REF
IN
AGND
47 F
NC
, V
12
3
inputs each have their own
1µF
REF
10
11
1
9
+
and AGND SENSE)
AGND
11
8
AD1876
47µF
V
AGND
REF
11
8
AD1876
AGND
V
REF
REV. A