AD7724ASTZ-REEL Analog Devices Inc, AD7724ASTZ-REEL Datasheet - Page 11

• Manufacturer Part Number: AD7724ASTZ-REEL
• Description: FOUR CHANNEL SIGMA DELTA MODULATOR I.C.
• Manufacturer: Analog Devices Inc
• Type: Modulatorr

Specifications of AD7724ASTZ-REEL

• Resolution (bits): 15 b
• Sampling Rate (per Second): 250k
• Data Interface: Serial
• Voltage Supply Source: Analog and Digital
• Voltage - Supply: 2.85 V ~ 5.25 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 48-LQFP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

An alternative circuit configuration for driving the differential
inputs to the AD7724 is shown in Figure 8.
A capacitor between the two input pins sources or sinks charge
to allow most of the charge needed by one input to be effectively
supplied by the other input. This minimizes undesirable 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 of the 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 digi-
tal 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 or polycarbonate are suitable. If ceramic
capacitors are used, they must have NPO dielectric.
Applying the Reference
The reference circuitry used in the AD7724 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 9. The
internal reference voltage is connected to REF1 via a 3 kΩ
resistor and is internally buffered to drive the analog modulator’s
switched capacitor DAC (REF2). When using the internal refer-
ence, connect 110 nF between REF1 and AGND. If the internal
reference is required to bias external circuits, use an external
precision op amp to buffer REF1.
110nF
Attenuation
f
REF2
REF1
3
dB
=
1 2
/
(
1V
π
=
R
20
100
100
EXT
R
R
log
4k
C
COMPARATOR
EXT



1 1
REFERENCE
/
)
BUFFER
REFERENCE
+
(
C
2.7nF
C
2.7nF
C
2.7nF
2.5V
f f
/
3
dB
)
2
VIN(+)
VIN(–)



SWITCHED-CAP
DAC REF
(1)
(2)
The AD7724 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. However, an error will be 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 con-
nected to REF2.
In all cases, since the REF2 voltage connects to the analog modu-
lator, a 110 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 10).
The AD780 is ideal to use as an external reference with the
AD7724. Figure 11 shows a suggested connection diagram.
1 F
NC = NO CONNECT
5V
22nF
110nF
REF2
1
2
3
4
NC
+V
TEMP
GND
IN
AD780
SELECT
MCLK
TRIM
V
O/P
OUT
NC
8
7
6
5
A
B
A
B
22 F
110nF
4pF
4pF
A
SWITCHED-CAP
DAC REF
B
AD7724
B
A
REF2
REF1