Data Sheet AD7475/AD7495
Rev. C | Page 15 of 24
When no amplifier is used to drive the analog input, the source
impedance should be limited to low values. The maximum
source impedance depends on the amount of total harmonic
distortion (THD) that can be tolerated. The THD increases as
the source impedance increases and performance degrades.
Figure 16 shows a graph of the total harmonic distortion vs.
source impedance for various analog input frequencies.
SOURCE IMPEDANCE (
Ω
)
–90
THD (dB)
1
100
–80
–70
–60
–50
10000
–40
f
IN
= 500kHz
f
IN
= 10kHz
f
IN
= 100kHz
f
IN
= 200kHz
10 1000
–30
–
20
–10
01684-B-016
Figure 16. THD vs. Source Impedance for Various Analog Input Frequencies
Figure 17 shows a graph of total harmonic distortion vs. analog
input frequency for various supply voltages while sampling at
1 MSPS with an SCLK of 20 MHz.
INPUT FREQUENCY (kHz)
THD (dB)
10 100
–95
–93
–91
–87
1000
–85
V
DD
= V
DRIVE
= 3.60V
V
DD
= V
DRIVE
= 2.70V
V
DD
= V
DRIVE
= 5.25V
V
DD
= V
DRIVE
= 4.75V
–83
–81
–
79
–77
–75
–89
01684-B-017
Figure 17. THD vs. Analog Input Frequency for Various Supply Voltages
Digital Inputs
The digital inputs applied to the AD7475/AD7495 are not limited
by the maximum ratings, which limit the analog inputs. Instead,
the digital inputs applied can go to 7 V and are not restricted by
the V
DD
+ 0.3 V limit as on the analog inputs. Another advantage
of SCLK and
CS
not being restricted by the V
DD
+ 0.3 V limit is
that power supply sequencing issues are avoided.
If
CS
or SCLK are applied before V
DD
, there is no risk of latch-up
as there would be on the analog inputs if a signal greater than
0.3 V were applied prior to V
DD
.
V
DRIVE
The AD7475/AD7495 also has the V
DRIVE
feature. This feature
controls the voltage at which the serial interface operates. V
DRIVE
allows the ADC to easily interface to both 3 V and 5 V processors.
For example, if the AD7475/AD7495 were operated with a V
DD
of 5 V, t h e V
DRIVE
pin could be powered from a 3 V supply. The
AD7475/AD7495 have better dynamic performance with a V
DD
of 5 V, while still being able to interface to 3 V digital devices.
Ensure V
DRIVE
does not exceed V
DD
by more than 0.3 V. (See the
Absolute Maximum Ratings section.)
Reference Section
Use an external reference source to supply the 2.5 V reference to
the AD7475. Errors in the reference source result in gain errors
in the AD7475 transfer function and add the specified full-scale
errors on the device. The AD7475 voltage reference input, REF IN,
has a dynamic input impedance. A small dynamic current is
required to charge the capacitors in the capacitive DAC during
the bit trials. This current is typically 50 µA for a 2.5 V reference.
Place a capacitor of at least 0.1 µF on the REF IN pin. Suitable
reference sources for the AD7475 are the AD780, AD680,
AD1582, ADR391, ADR381, ADR431, and ADR03.
The AD7495 contains an on-chip 2.5 V reference. As shown in
Figure 18, the voltage that appears at the REF OUT pin internally
buffers before applied to the ADC; the output impedance of this
buffer is typically 10 Ω. The reference is capable of sourcing up to
2 mA. Decouple the REF OUT pin to AGND using a 100 nF or
greater capacitor.
If the 2.5 V internal reference is used to drive another device
that is capable of glitching the reference at critical times, then
the reference has to be buffered before driving the device. To
ensure optimum performance of the AD7495, it is recommended
that the internal reference not be over driven. If an ADC with
external reference capability is required, use the AD7475.
V
REF OUT
25Ω
40kΩ
160kΩ
01684-B-018
Figure 18. AD7495 Reference Circuit