AD7720BRU Analog Devices Inc, AD7720BRU Datasheet - Page 11

• Manufacturer Part Number: AD7720BRU
• Description: IC MODULATOR SIGMA-DELTA 28TSSOP
• Manufacturer: Analog Devices Inc

Specifications of AD7720BRU

• Rohs Status: RoHS non-compliant
• Number Of Bits: 16
• Sampling Rate (per Second): 12.5M
• Data Interface: Parallel
• Number Of Converters: 1
• Power Dissipation (max): 215mW
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-TSSOP (0.173", 4.40mm Width)

REV. 0
A capacitor between the two input pins sources or sinks charge
to allow most of the charge that is needed by one input to be
effectively supplied by the other input. This minimizes undesir-
able charge transfer from the analog inputs to and from ground.
The series resistor isolates the operational amplifier from the
current spikes created during the sampling process and provides
a pole for antialiasing. The 3 dB cutoff frequency (f
antialias filter is given by Equation 1, and the attenuation of the
filter is given by Equation 2.
The choice of the filter cutoff frequency will depend on the
amount of roll off that is acceptable in the passband of the
digital filter and the required attenuation at the first image
frequency.
The capacitors used for the input antialiasing circuit must have
low dielectric absorption to avoid distortion. Film capacitors
such as Polypropylene, Polystyrene or Polycarbonate are suitable.
If ceramic capacitors are used, they must have NPO dielectric.
Applying the Reference
The reference circuitry used in the AD7720 includes an on-chip
+2.5 V bandgap reference and a reference buffer circuit. The
block diagram of the reference circuit is shown in Figure 25.
The internal reference voltage is connected to REF1 via a
3 k resistor and is internally buffered to drive the analog
modulator’s switched capacitor DAC (REF2). When using the
internal reference, connect 100 nF between REF1 and AGND.
If the internal reference is required to bias external circuits, use
an external precision op amp to buffer REF1.
100nF
f
Attenuation = 20 log 1/ 1
3 dB
REF2
REF1
Figure 24. Differential Input with Antialiasing
Figure 25. Reference Circuit Block Diagram
= 1/(2
1V
R
100
100
EXT
R
R
C
3k
COMPARATOR
EXT
)
REFERENCE
BUFFER
REFERENCE
C
2.7nF
C
2.7nF
C
2.7nF
2.5V
f / f
3 dB
VIN(–)
VIN(+)
2
SWITCHED-CAP
DAC REF
3 dB
) of the
(1)
(2)
–11–
The AD7720 can operate with its internal reference or an external
reference can be applied in two ways. An external reference can
be connected to REF1, overdriving the internal reference. How-
ever, there will be an error introduced due to the offset of the
internal buffer amplifier. For lowest system gain errors when
using an external reference, REF1 is grounded (disabling the
internal buffer) and the external reference is connected to REF2.
In all cases, since the REF2 voltage connects to the analog
modulator, a 220 nF capacitor must connect directly from
REF2 to AGND. The external capacitor provides the charge
required for the dynamic load presented at the REF2 pin
(Figure 26).
The AD780 is ideal to use as an external reference with the
AD7720. Figure 27 shows a suggested connection diagram.
Input Circuits
Figures 28 and 29 show two simple circuits for bipolar mode
operation. Both circuits accept a single-ended bipolar signal
source and create the necessary differential signals at the input
to the ADC.
The circuit in Figure 28 creates a 0 V to 2.5 V signal at the
VIN(+) pin to form a differential signal around an initial bias
voltage of 1.25 V. For single-ended applications, best THD
performance is obtained with VIN(–) set to 1.25 V rather than
2.5 V. The input to the AD7720 can also be driven differen-
tially with a complementary input as shown in Figure 29.
In this case, the input common-mode voltage is set to 2.5 V.
The 2.5 V p-p full-scale differential input is obtained with a
1.25 V p-p signal at each input in antiphase. This configuration
minimizes the required output swing from the amplifier circuit
and is useful for single supply applications.
1 F
Figure 27. External Reference Circuit Connection
+5V
22nF
220nF
REF2
Figure 26. REF2 Equivalent Circuit
1
2
3
4
+V
NC
TEMP
GND
IN
AD780
SELECT
MCLK
TRIM
V
O/P
OUT
NC
8
7
6
5
A
B
A
B
22 F
220nF
4pF
4pF
A
SWITCHED-CAP
DAC REF
B
B
A
AD7720
REF2
REF1