EVAL-AD7678CB Analog Devices Inc, EVAL-AD7678CB Datasheet - Page 18

• Manufacturer Part Number: EVAL-AD7678CB
• Description: BOARD EVALUATION FOR AD7678
• Manufacturer: Analog Devices Inc
• Series: PulSAR®r

Specifications of EVAL-AD7678CB

• Number Of Adc's: 1
• Number Of Bits: 18
• Sampling Rate (per Second): 100k
• Data Interface: Serial, Parallel
• Inputs Per Adc: 1 Differential
• Input Range: ±VREF
• Power (typ) @ Conditions: 31mW @ 100kSPS
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Utilized Ic / Part: AD7678
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

AD7678
The
ate 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
of 1 is present. The
where high frequency (above 100 kHz) performance is not
required. In gain of 1 applications, it requires an 82 pF
compensation capacitor. The
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 24. 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.
Voltage Reference
The AD7678 allows the use of an external voltage reference 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


ANALOG INPUT
0V TO 4.096V)
(UNIPOLAR
AD8021
AD8022
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).
Figure 24. Single-Ended-to-Differential Driver Circuit
8.25k 
1.82k 
meets these requirements and is usually appropri-
could be used if a dual version is needed and gain
REF
with midscale at V
U1
(Internal Reference Buffer Used)
AD829
100nF
REF
AD8021
10pF
590 
590 
, this configuration will produce a
U2
is an alternative in applications
AD8021
10pF
AD8610
REF
is another option when
/2.
IN+
IN– REF
AD7678
10  F
REFBUFIN
03084-0-030
2.5V
Rev. A | Page 18 of 28
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 AD7678 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 21. The AD7678 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 25.
65
60
55
50
45
40
1
Figure 25. PSRR vs. Frequency
10
FREQUECY (kHz)
100
1000
03084-0-031
10000