AD7674ACPZRL Analog Devices Inc, AD7674ACPZRL Datasheet - Page 19

• Manufacturer Part Number: AD7674ACPZRL
• Description: 18-Bit,800kSPSSARADC
• Manufacturer: Analog Devices Inc
• Series: PulSAR®r

Specifications of AD7674ACPZRL

• Number Of Bits: 18
• Sampling Rate (per Second): 800k
• Data Interface: Serial, Parallel
• Number Of Converters: 1
• Power Dissipation (max): 138mW
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 48-VFQFN, CSP Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With: EVAL-AD7674CBZ - BOARD EVALUATION FOR AD7674
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

•
The AD8021 meets these requirements and is usually
appropriate for almost all applications. The AD8021 needs a
10 pF external compensation capacitor, which should have good
linearity as an NPO ceramic or mica type.
The AD8022 could be used if a dual version is needed and gain
of 1 is present. The AD829 is an alternative in applications
where high frequency (above 100 kHz) performance is not
required. In gain of 1 applications, it requires an 82 pF
compensation capacitor. The AD8610 is another option when
low bias current is needed in low frequency applications.
Single-to-Differential Driver
For applications using unipolar analog signals, a single-ended-
to-differential driver will allow for a differential input into the
part. The schematic is shown in Figure 31. When provided an
input signal of 0 to V
differential ±V
If the application can tolerate more noise, the AD8138
differential driver can be used.
ANALOG INPUT
0V TO 4.096V)
(UNIPOLAR
where:
f
(26 MHz) or the cutoff frequency of the input filter, if used.
N is the noise factor of the amplifiers (1 if in buffer
configuration).
e
nV/√Hz.
For instance, for a driver with an equivalent input noise of
2 nV/√Hz (e.g., AD8021) configured as a buffer, thus with
a noise gain of +1, the SNR degrades by only 0.34 dB with
the filter in Figure 27, and by 1.8 dB without it.
The driver needs to have a THD performance suitable to
that of the AD7674.
SNR
–3dB
N
is the equivalent input noise voltage of each op amp in
is the –3 dB input bandwidth in MHz of the AD7674
LOSS
Figure 31. Single-Ended-to-Differential Driver Circuit
8.25kΩ
1.82k Ω
=
REF
20
with midscale at V
U1
log
(Internal Reference Buffer Used)
100nF
REF
AD8021
⎛
⎜
⎜
⎜
⎜
⎜
⎝
10pF
590Ω
590Ω
, this configuration will produce a
U2
625
AD8021
10pF
+
π
2.7nF
15 Ω
15 Ω
f
25
–
3dB
REF
(
/2.
Ne
2.7nF
N
IN+
IN– REF
2 )
⎞
⎟
⎟
⎟
⎟
⎟
⎠
AD7674
10 μ F
REFBUFIN
03083-0-031
2.5V
Rev. A | Page 19 of 28
Voltage Reference
The AD7674 allows the use of an external voltage reference
either with or without the internal reference buffer.
Using the internal reference buffer is recommended when
sharing a common reference voltage between multiple ADCs is
desired.
However, the advantages of using the external reference voltage
directly are:
•
•
To use the internal reference buffer, PDBUF should be LOW. A
2.5 V reference voltage applied on the REFBUFIN input will
result in a 4.096 V reference on the REF pin.
In both cases, the voltage reference input REF has a dynamic
input impedance and therefore requires an efficient decoupling
between REF and REFGND inputs, The decoupling consists of
a low ESR 47 μF tantalum capacitor connected to the REF and
REFGND inputs with minimum parasitic inductance.
Care should also be taken with the reference temperature
coefficient of the voltage reference, which directly affects the
full-scale accuracy if this parameter matters. For instance, a
±4 ppm/°C temperature coefficient of the reference changes the
full scale by ±1 LSB/°C.
Power Supply
The AD7674 uses three sets of power supply pins: an analog 5 V
supply (AVDD), a digital 5 V core supply (DVDD), and a digital
output interface supply (OVDD). The OVDD supply defines
the output logic level and allows direct interface with any logic
working between 2.7 V and DVDD + 0.3 V. To reduce the
number of supplies needed, the digital core (DVDD) can be
supplied through a simple RC filter from the analog supply, as
shown in Figure 27. The AD7674 is independent of power
supply sequencing once OVDD does not exceed DVDD by
more than 0.3 V, and is therefore free from supply voltage
induced latch-up. Additionally, it is very insensitive to power
supply variations over a wide frequency range, as shown in
Figure 32.
The SNR and dynamic range improvement (about 1.7 dB)
resulting from the use of a reference voltage very close to
the supply (5 V) instead of a typical 4.096 V reference
when the internal buffer is used
The power saving when the internal reference buffer is
powered down (PDBUF High)
AD7674