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AD7450ARZ-REEL7

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P23
–12–
AD7450
REFERENCE = 1.25V
1.25V p-p
COMMON-MODE (CM)
CM
MIN
= 0.625V
CM
MAX
= 4.42V
REFERENCE = 2.5V
2.5V p-p
COMMON-MODE (CM)
CM
MIN
= 1.25V
CM
MAX
= 3.75V
V
IN
–
V
IN
V
IN
–
V
IN
Figure 10. Examples of the Analog Inputs to V
IN+
and V
IN–
for Different Values of V
REF
for V
DD
= 5 V
Analog Input Structure
Figure 11 shows the equivalent circuit of the analog input struc-
ture of the AD7450. The four diodes provide ESD protection
for the analog inputs. Care must be taken to ensure that the
analog input signals never exceed the supply rails by more than
300 mV. This will cause these diodes to become forward biased
and start conducting into the substrate. These diodes can conduct
up to 10 mA without causing irreversible damage to the part.
The capacitors, C1, in Figure 11 are typically 4 pF and can prima-
rily
be attributed to pin capacitance. The resistors are lumped
components made up of the ON resistance of the switches. The
value of these resistors is typically about 100 Ω. The capacitors,
C2, are the ADC’s sampling capacitors and have a capacitance
of 16 pF typically.
For ac applications, removing high-frequency components from
the analog input signal is recommended by the use of an RC
low-pass filter on the relevant analog input pins. In applications
where harmonic distortion and signal-to-noise ratio are critical,
the analog input should be driven from a low impedance source.
Large source impedances will significantly affect the ac perfor-
mance of the ADC. This may necessitate the use of an input
buffer amplifier. The choice of the op amp will be a function of
the particular application.
V
DD
C1
D
D
V
IN+
R1
C2
V
IN–
R1
C2
V
DD
D
D
C1
Figure 11. Equivalent Analog Input Circuit
Conversion Phase—Switches Open
Track Phase—Switches Closed
When no amplifier is used to drive the analog input, the source
impedance should be limited to values lower than 1 kΩ. The
maximum source
impedance will depend on the amount of
total harmonic
distortion (THD) that can be tolerated. The THD
will increase
as the source impedance increases and the perfor-
mance will degrade.
Figure 12 shows a graph of the THD versus
the analog input signal frequency for different source impedances.
INPUT FREQUENCY – kHz
–74
–70
–72
–76
–78
–82
–80
10 1000100
THD – dBs
T
A
= 25C
V
DD
= 5V
R
IN
= 1k
V
DD
= 3V
R
IN
= 1k
V
DD
= 3V
R
IN
= 100
V
DD
= 5V
R
IN
= 100
Figure 12. THD vs. Analog Input Frequency for
Various Source Impedances for V
DD
= 5 V and 3 V
Figure 13 shows a graph of the THD versus the analog input
frequency for V
DD
of 5 V ± 5% and 3 V ± 10%, while sampling
at 1 MSPS and 833 kSPS with a SCLK of 18 MHz and
15 MHz, respectively. In this case, the source impedance is 10 Ω.
INPUT FREQUENCY – kHz
–95
–70
–60
–65
–75
–80
–90
–85
10 1000100
THD – dBs
T
A
= 25C
V
DD
= 3.3V
V
DD
= 2.7V
V
DD
= 4.75V
V
DD
= 5.25V
Figure 13. THD vs. Analog Input Frequency for 3 V
±
10%
and 5 V
±
5% Supply Voltages
DRIVING DIFFERENTIAL INPUTS
Differential operation requires that V
IN+
and V
IN–
be simulta-
neously driven with two equal signals that are 180
o
out of phase.
The common mode must be set up externally and has a range
that is determined by V
REF
, the power supply, and the particular
amplifier used to drive the analog inputs (see Figures 8 and 9).
Differential modes of operation with either an ac or dc input
provide the best THD performance over a wide frequency range.
Since not all applications have a signal preconditioned for
differential operation, there is often a need to perform single-
ended-to-differential conversion.
Rev. A
AD7450
–13–
Differential Amplifier
An ideal method of applying differential drive to the AD7450 is to
use a differential amplifier, such as the AD8138. This part can be
used as a single-ended-to-differential amplifier or as a differential-
to-differential amplifier. In both cases, the analog input needs to
be bipolar. It also provides common-mode level shifting and buffer-
ing of the bipolar input signal. Figure 14 shows how the AD8138
can be used as a single-ended-to-differential amplifier. The positive
and negative outputs of the AD8138 are connected to the respective
inputs on the ADC via a pair of series resistors to minimize the
effects of switched capacitance on the front end of the ADC.
The RC low-pass filter on each analog input is recommended in
ac applications to remove the high-frequency components of the
analog input. The architecture of the AD8138 results in outputs
that are highly balanced over a wide frequency range without
requiring tightly matched external components.
If the analog input source being used has zero impedance then all
four resistors (Rg1, Rg2, Rf1, and Rf2) should be the same. If the
source has a 50 Ω impedance and a 50 Ω termination, for example,
the value of Rg2 should be increased by 25 Ω to balance this paral-
lel impedance on the input and thus ensure that both the positive
and negative analog inputs have the same gain (see Figure 14).
The outputs of the amplifier are perfectly matched, balanced
differential outputs of identical amplitude and exactly 180
o
out
of phase.
The AD8138 is specified with 3 V, 5 V, and ± 5 V power supplies,
but the best results are obtained when it is supplied by ± 5 V.
A lower cost device that could also be used in this configuration
with slight differences in characteristics to the AD8138, but with
similar performance and operation, is the AD8132.
Op Amp Pair
An op amp pair can be used to directly couple a differential
signal to the AD7450. The circuit configurations shown in
Figures
15a and 15b show how a dual op amp can be used to
convert
a single-ended signal into a differential signal for both a
bipolar and a unipolar input signal, respectively.
The voltage applied to Point A sets up the common-mode voltage.
In both diagrams, it is connected in some way to the reference,
but any value in the common-mode range can be input here to
set up the common mode. Examples of suitable dual op amps
that could be used in this configuration to provide differential
drive to the AD7450 are the AD8042, AD8056, and AD8022.
Care must be taken when choosing the op amp, since the selec-
tion will depend on the required power supply and the system
performance objectives. The driver circuits in Figure 15a and
Figure 15b are optimized for dc coupling applications requiring
optimum distortion performance.
The differential op amp driver circuit in Figure 15a is configured
to convert and level shift a single-ended, ground
referenced
(bipolar) signal to a differential signal centered
at the V
REF
level
of the ADC.
V+
V–
V+
V–
27
27
390
220
220
10k
EXTERNAL
V
REF
220
V
DD
V
IN+
V
IN–
AD7450
220
20k
0.1F
A
V
REF
2 V
REF
p-p
GND
Figure 15a. Dual Op Amp Circuit to Convert a
Single-Ended Bipolar Input into a Differential Input
2.5V
3.75V
1.25V
Rs*
Rs*
Rf2
2.5V
3.75V
1.25V
EXTERNAL
V
REF
(2.5V)
V
REF
V
IN+
AD7450
V
IN–
AD8138
C
*
C
*
*MOUNT AS CLOSE TO THE AD7450
AS POSSIBLE AND ENSURE HIGH
PRECISION Rs AND Cs ARE USED
Rs – 50R; C – 1nF;
Rg1 = Rf1 = Rf2 = 499R; Rg2 = 523R
Rf1
Rg1
V
OCM
51R
Rg2
GND
+2.5V
–2.5V
Figure 14. Using the AD8138 as a Single-Ended-to-Differential Amplifier
Rev. A
–14–
AD7450
The circuit configuration shown in Figure 15b converts a unipolar,
single-ended signal into a differential signal.
V+
V–
V+
V–
2 V
REF
p-p
VREF
27
27
390
220
10k
EXTERNAL
V
REF
220
V
DD
V
IN+
V
IN–
AD7450
220
0.1F
A
V
REF
GND
Figure 15b. Dual Op Amp Circuit to Convert a
Single-Ended Unipolar Input into a Differential Input
RF Transformer
In systems that do not need to be dc-coupled, an RF transformer
with a center tap offers a good solution for generating differential
inputs. Figure 16 shows how a transformer is used for single-
ended-to-differential conversion. It provides the benefits of
operating the ADC in the differential mode without contributing
additional noise and distortion. An RF transformer also has the
benefit of providing electrical isolation between the signal source
and the ADC. A transformer can be used for most ac applications.
The center tap is used to shift the differential signal to the
common-mode level required. In this case, it is connected to the
reference so the common-mode level is the value of the reference.
C
EXTERNAL
V
REF
(2.5V)
R
R
R
V
REF
V
IN+
AD7450
V
IN–
2.5V
3.75V
1.25V
2.5V
3.75V
1.25V
Figure 16. Using an RF Transformer to Generate
Differential Inputs
REFERENCES SECTION
An external reference source is required to supply the reference to the
AD7450. This reference input can range from 100 mV to 3.5 V. With
a 5 V power supply, the specified reference is 2.5 V and the maximum
reference is 3.5 V. With a 3.3 V power supply, the specified refer-
ence is 1.25 V and the maximum reference is 2.4 V. In both cases,
the reference is functional from 100 mV. It is important to ensure
that, when choosing the reference value for a particular application,
the maximum analog input range (V
IN
max) is never greater than
V
DD
+ 0.3 V to comply with the maximum ratings of the part. The
following two examples calculate the maximum V
REF
input that can be
used when operating the AD7450 at V
DD
of 5 V and 3.3 V, respectively.
Example 1:
VV
IN DD
max .=+03
VVV
IN REF REF
max =+ 2
If V V
DD
= 5
ThenV V
IN
max .= 53
Therefore V V
REF
3253×=.
VV
REF
max .= 35
Therefore, when operating at V
DD
= 5 V, the value of V
REF
can
range from 100 mV to a maximum value of 3.5 V. When V
DD
=
4.75 V, V
REF
max = 3.37 V.
Example 2:
VV
IN DD
max .=+03
VVV
IN REF REF
max =+ 2
If V V
DD
= 33.
ThenV V
IN
max .= 36
Therefore V V
REF
3236×=.
VV
REF
max .= 24
Therefore, when operating at V
DD
= 3.3 V, the value of V
REF
can range from 100 mV to a maximum value of 2.4 V. When
V
DD
= 2.7 V, V
REF
max = 2 V.
These examples show that the maximum reference applied to
the AD7450 is directly dependant on the value of V
DD
.
The performance of the part at different reference values is shown
in TPC 8 to TPC 12 and in TPC 15. The value of the reference
sets the analog input span and the common-mode voltage range.
Errors in the reference source will result in gain errors in the
AD7450 transfer function and will add to specified
full-scale
errors
on the part. A capacitor of 0.1 µF should be used to
decouple
the V
REF
pin to GND. Table I lists examples of suitable voltage
references to be used that are available from Analog Devices, and
Figure 17 shows a typical connection diagram for the V
REF
pin.
Table I. Examples of Suitable Voltage References
Output Initial Operating
Reference Voltage Accuracy (% Max) Current (A)
AD589 1.235 1.2–2.8 50
AD1580 1.225 0.08–0.8 50
REF192 2.5 0.08–0.4 45
REF43 2.5 0.06–0.1 600
AD780 2.5 0.04–0.2 1000
V
REF
AD7450*
V
DD
0.1F
1
2
3
4
5
6
7
8
V
IN
TEMP
GND
TRIM
V
OUT
O/P SEL
NC
NC
NC
NC
V
DD
0.1F
0.1F
10nF
*ADDITIONAL PINS OMITTED FOR CLARITY
AD780
NC = NO CONNECT
2.5V
Figure 17. Typical V
REF
Connection Diagram for V
DD
= 5 V
Rev. A
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P23

AD7450ARZ-REEL7

Mfr. #:
Buy AD7450ARZ-REEL7
Manufacturer:
Analog Devices Inc.
Description:
Analog to Digital Converters - ADC 3V/5V DIFF INPUT 12 BIT SAR IC
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New from this manufacturer.
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