EVAL-AD7650CBZ Analog Devices Inc, EVAL-AD7650CBZ Datasheet - Page 12

• Manufacturer Part Number: EVAL-AD7650CBZ
• Description: BOARD EVALUATION FOR AD7650
• Manufacturer: Analog Devices Inc

Specifications of EVAL-AD7650CBZ

• Number Of Adc's: 1
• Number Of Bits: 16
• Sampling Rate (per Second): 570k
• Data Interface: Serial, Parallel
• Inputs Per Adc: 1 Differential
• Input Range: ±VREF
• Power (typ) @ Conditions: 150mW @ 570kSPS
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Utilized Ic / Part: AD7650
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD7650
Analog Input
Figure 6 shows an equivalent circuit of the input structure of the
AD7650.
The two diodes, D1 and D2, provide ESD protection for the
analog inputs IN+ and IN–. Care must be taken to ensure that the
analog input signal never exceeds the supply rails by more than
0.3 V. This will cause these diodes to become forward-biased and
start conducting current. These diodes can handle a forward-
biased current of 100 mA maximum. For instance, these conditions
could eventually occur when the input buffer’s (U1) supplies are
different from AVDD. In such case, an input buffer with a short
circuit current limitation can be used to protect the part.
This analog input structure allows the sampling of the differential
signal between IN+ and IN–. Unlike other converters, the IN–
input is sampled at the same time as the IN+ input. By using
this differential input, small signals common to both inputs are
rejected. For instance, by using IN– to sense a remote signal
ground, difference of ground potentials between the sensor and
the local ADC ground are eliminated.
During the acquisition phase, the impedance of the analog input
IN+ can be modeled as a parallel combination of capacitor C1
and the network formed by the series connection of R1 and C2.
Capacitor C1 is primarily the pin capacitance. The resistor R1 is
typically 140 Ω and is a lumped component made up of some
serial resistors and the on resistance of the switches. The capacitor
C2 is typically 60 pF and is mainly the ADC sampling capacitor.
During the conversion phase, where the switches are opened,
the input impedance is limited to C1. The R1, C2 makes a one-
pole low-pass filter that reduces undesirable aliasing effect and
limits the noise.
When the source impedance of the driving circuit is low, the
AD7650 can be driven directly. Large source impedances will
significantly affect the ac performances, especially the total
harmonic distortion.
Driver Amplifier Choice
Although the AD7650 is easy to drive, the driver amplifier needs
to meet at least the following requirements:
• The driver amplifier and the AD7650 analog input circuit
must be able together to settle for a full-scale step the capacitor
array at a 16-bit level (0.0015%). In the amplifier’s data sheet,
the settling at 0.1% to 0.01% is more commonly specified. It
could significantly differ from the settling time at 16-bit level
and it should therefore be verified prior to the driver selection.
The tiny op amp AD8021, which combines ultralow noise and
a high-gain bandwidth, meets this settling time requirement
even when used with high gain up to 13.
OR IN–
AGND
IN+
C1
AVDD
D1
D2
R1
C2
• The noise generated by the driver amplifier needs to be kept
• The driver needs to have a THD performance suitable to that
The AD8021 meets these requirements and is usually appropri-
ate for almost all applications. The AD8021 needs an external
compensation capacitor of 10 pF. This capacitor should have
good linearity as an NPO ceramic or mica type.
The AD8022 could also be used where dual version is needed
and gain of 1 is used.
The AD829 is another alternative where high-frequency (above
100 kHz) performance is not required. In gain of 1, it requires
an 82 pF compensation capacitor.
The AD8610 is another option where low bias current is needed
in low-frequency applications.
Voltage Reference Input
The AD7650 uses an external 2.5 V voltage reference. The volt-
age reference input REF of the AD7650 has a dynamic input
impedance. Therefore, it should be driven by a low impedance
source with an efficient decoupling between REF and REFGND
inputs. This decoupling depends on the choice of the voltage
reference, but usually consists of a low ESR tantalum capacitor
connected to the REF and REFGND inputs with minimum para-
sitic inductance. 47 µF is an appropriate value for tantalum capacitor
when used with one of the recommended reference voltages:
• The low-noise, low temperature drift ADR421 and AD780
• The low-power ADR291 voltage reference.
• The low-cost AD1582 voltage reference.
For applications using multiple AD7650s, it is more effective to
buffer the reference voltage with a low-noise, very stable op amp
such as the AD8031.
Care should also be taken with the reference temperature coeffi-
cient of the voltage reference which directly affects the full-scale
accuracy if this parameter matters. For instance, a ± 15 ppm/°C
tempco of the reference changes the full scale by ± 1 LSB/°C.
Note that V
be increased to AVDD –1.85 V. Since the input range is defined
in terms of V
make it a 0 V to 3 V input range with a reference voltage of 3 V.
The AD780 can be selected with a 3 V reference voltage.
Power Supply
The AD7650 uses three sets of power supply pins: an analog 5 V
supply AVDD, a digital 5 V core supply DVDD, and a digital
input/output interface supply OVDD. The OVDD supply allows
direct interface with any logic working between 2.7 V and 5.25 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 5. The AD7650 is independent of
power supply sequencing and thus free from supply voltage
induced latchup.
voltage references.
as low as possible to preserve the SNR and transition noise
performance of the AD7650. The noise coming from the
driver is filtered by the AD7650 analog input circuit one-pole
low-pass filter made by R1 and C2 or the external filter if any
are used.
of the AD7650.
REF
REF
, as mentioned in the specification table, could
, this would essentially increase the range to