EVAL-CONTROLBOARD AD [Analog Devices], EVAL-CONTROLBOARD Datasheet - Page 13

• Manufacturer Part Number: EVAL-CONTROLBOARD
• Description: 16-Bit, 100 kSPS CMOS ADC
• Manufacturer: AD [Analog Devices]

When the source impedance of the driving circuit is low, the
AD7660 can be driven directly. Large source impedances will
significantly affect the ac performances, especially the total
harmonic distortion. The maximum source impedance depends
on the amount of total harmonic distortion (THD) that can be
tolerated. The THD degrades in function of the source imped-
ance and the maximum input frequency as shown in Figure 8.
Driver Amplifier Choice
Although the AD7660 is easy to drive, the driver amplifier needs
to meet at least the following requirements:
• The driver amplifier and the AD7660 analog input circuit
• The noise generated by the driver amplifier needs to be kept
• The driver needs to have a THD performance suitable to
The AD8519, OP162, or the OP184 meet these requirements
and are usually appropriate for almost all applications. As an
alternative, in very high-speed and noise-sensitive applications,
the AD829 with an external compensation capacitor of 82 pF
can be used. This capacitor should have good linearity as an
NPO ceramic or mica type. Moreover, the use of a noninverting
+1 gain arrangement is recommended and helps to obtain the
best signal-to-noise ratio.
Voltage Reference Input
The AD7660 uses an external 2.5 V voltage reference. The
voltage reference input REF of the AD7660 has a dynamic
input impedance. Therefore, it should be driven by a low
impedance source with an efficient decoupling between REF
have to be able together to settle for a full-scale step the
capacitor array at a 16-bit level (0.0015%). For instance,
operation at the maximum throughput of 100 kSPS requires
a minimum gain bandwidth product of 5 MHz.
as low as possible in order to preserve the SNR and transi-
tion noise performance of the AD7660. The noise coming
from the driver is filtered by the AD7660 analog input circuit
one-pole low-pass filter made by R1 and C2. For instance, a
driver with an equivalent input noise of 7 nV/√Hz like the
AD8519 and configured as a buffer, thus with a noise gain of
+1, degrades the SNR by only 0.2 dB.
that of the AD7660. TPC 8 gives the THD versus frequency
that the driver should preferably exceed.
–100
–70
–75
–80
–85
–90
–95
1
INPUT FREQUENCY – kHz
10
R
R
S
S
R
R
S
= 500
= 100
S
= 20
= 50
100
and REFGND inputs. This decoupling depends on the choice
of the voltage reference but, usually consists of a low ESR tanta-
lum capacitor and a 100 nF ceramic capacitor. Appropriate
value for the tantalum capacitor is 47 µF with the low-cost,
low-power ADR291 voltage reference or with the low-noise,
low-drift AD780 voltage reference. For applications using
multiple AD7660s, it is more effective to buffer the reference
voltage with a low-noise, very stable op amp like 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.
Power Supply
The AD7660 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 6. The AD7660 is inde-
pendent of power supply sequencing and thus free from supply
voltage induced latchup. Additionally, it is very insensitive to
power supply variations over a wide frequency range as shown in
Figure 9.
POWER DISSIPATION VS. THROUGHPUT
The AD7660 automatically reduces its power consumption at
the end of each conversion phase. During the acquisition phase,
the operating currents are very low which allows a significant
power saving when the conversion rate is reduced as shown in
Figure 10. This feature makes the AD7660 ideal for very low-
power battery applications. It should be noted that the digital
interface remains active even during the acquisition phase. To
reduce the operating digital supply currents even further, the
digital inputs need to be driven close to the power rails (i.e.,
DVDD and DGND for all inputs except EXT/INT, INVSYNC,
INVSCLK, RDC/SDIN, and OVDD or OGND for the last
four inputs.
–50
–55
–60
–65
–70
–75
–80
1k
INPUT FREQUENCY – Hz
10k
100k
AD7660
1M