AD AD7457 Low power, pseudo differential, 100 ksps 12-bit adc in an 8-lead sot-23 Datasheet

Low Power, Pseudo Differential, 100 kSPS
12-Bit ADC in an 8-Lead SOT-23
AD7457
FEATURES
FUNCTIONAL BLOCK DIAGRAM
Specified for VDD of 2.7 V to 5.25 V
Low power:
0.9 mW max at 100 kSPS with VDD = 3 V
3 mW max at 100 kSPS with VDD = 5 V
Pseudo differential analog input
Wide input bandwidth:
70-dB SINAD at 30 kHz input frequency
Flexible power/serial clock speed management
No pipeline delays
High speed serial interface—SPI®/QSPI™/
MICROWIRE™/DSP compatible
Automatic power-down mode
8-lead SOT-23 package
VDD
VIN+
12-BIT
SUCCESSIVE
APPROXIMATION
ADC
T/H
VIN–
VREF
SCLK
SDATA
AD7457
CONTROL LOGIC
CS
Transducer interface
Battery-powered systems
Data acquisition systems
Portable instrumentation
03157-0-013
APPLICATIONS
GND
Figure 1.
GENERAL DESCRIPTION
PRODUCT HIGHLIGHTS
The AD7457 is a 12-bit, low power, successive approximation
(SAR) analog-to-digital converter that features a pseudo
differential analog input. This part operates from a single 2.7 V
to 5.25 V power supply and features throughput rates of up to
100 kSPS.
1.
Operation with 2.7 V to 5.25 V power supplies.
2.
Low power consumption. With a 3 V supply, the AD7457
offers 0.9 mW maximum power consumption for a
100 kSPS throughput rate.
The part contains a low noise, wide bandwidth, differential
track-and-hold amplifier (T/H) that can handle input
frequencies in excess of 1 MHz. The reference voltage for the
AD7457 is applied externally to the VREF pin and can range from
100 mV to VDD, depending on what suits the application.
3.
Pseudo differential analog input.
4.
Flexible power/serial clock speed management. The
conversion rate is determined by the serial clock, allowing
the power to be reduced as the conversion time is reduced
through the serial clock speed increase. Automatic powerdown after conversion allows the average power
consumption to be reduced.
5.
Variable voltage reference input.
6.
No pipeline delay.
7.
Accurate control of the sampling instance via the CS input
and once-off conversion control.
8.
ENOB > 10 bits typically with 500 mV reference.
The conversion process and data acquisition are controlled
using CS and the serial clock, allowing the device to interface
with microprocessors or DSPs.
The SAR architecture of this part ensures that there are no
pipeline delays.
The AD7457 uses advanced design techniques to achieve very
low power dissipation.
Rev. 0
Information furnished by Analog Devices is believed to be accurate and reliable.
However, no responsibility is assumed by Analog Devices for its use, nor for any
infringements of patents or other rights of third parties that may result from its use.
Specifications subject to change without notice. No license is granted by implication
or otherwise under any patent or patent rights of Analog Devices. Trademarks and
registered trademarks are the property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700
www.analog.com
Fax: 781.326.8703
© 2003 Analog Devices, Inc. All rights reserved.
AD7457
TABLE OF CONTENTS
Specifications..................................................................................... 3
Analog Input ............................................................................... 12
Timing Specifications....................................................................... 5
Analog Input Structure.............................................................. 12
Absolute Maximum Ratings............................................................ 6
Digital Inputs .............................................................................. 13
ESD Caution.................................................................................. 6
Reference Section ....................................................................... 13
Pin Configuration and Functional Descriptions.......................... 7
Serial Interface ............................................................................ 13
Terminology ...................................................................................... 8
Power Consumption .................................................................. 14
Typical Performance Characteristics ............................................. 9
Microprocessor Interfacing....................................................... 14
Theory of Operation ...................................................................... 11
Application Hints ........................................................................... 16
Circuit Information.................................................................... 11
Grounding and Layout .............................................................. 16
Converter Operation.................................................................. 11
Outline Dimensions ....................................................................... 17
ADC Transfer Function............................................................. 11
Ordering Guide .......................................................................... 17
Typical Connection Diagram ................................................... 11
REVISION HISTORY
Revision 0: Initial Version
Rev. 0 | Page 2 of 20
AD7457
SPECIFICATIONS
VDD = 2.7 V to 5.25 V, fSCLK = 10 MHz, fS = 100 kSPS, VREF = 2.5 V, TA = TMIN to TMAX, unless otherwise noted.
Table 1.
Parameter
DYNAMIC PERFORMANCE
Signal to Noise Ratio (SNR)2
Signal to (Noise + Distortion) (SINAD)2
Total Harmonic Distortion (THD)2
Peak Harmonic or Spurious Noise2
Intermodulation DIstortion (IMD)2
Second Order Terms
Third Order Terms
Aperture Delay2
Aperture Jitter2
Full-Power Bandwidth2,3
DC ACCURACY
Resolution
Integral Nonlinearity (INL)2
Differential Nonlinearity (DNL)2
Offset Error2
Gain Error2
ANALOG INPUT
Full-Scale Input Span
Absolute Input Voltage
VIN+
VIN– 4
DC Leakage Current
Input Capacitance
REFERENCE INPUT
VREF Input Voltage
DC Leakage Current
VREF Input Capacitance
LOGIC INPUTS
Input High Voltage, VINH
Input Low Voltage, VINL
Input Current, IIN
Input Capacitance, CIN6
LOGIC OUTPUTS
Output High Voltage, VOH
Output Low Voltage, VOL
Floating-State Leakage Current
Floating-State Output Capacitance6
Output Coding
CONVERSION RATE
Conversion Time
Track-and-Hold Acquisition Time2
Throughput Rate
B Version1
Unit
71
70
−75
−75
dB min
dB min
dB max
dB max
−80
−80
5
50
20
2.5
dB typ
dB typ
ns typ
ps typ
MHz typ
MHz typ
Guaranteed no missed codes to 12 bits
12
±1
±0.95
±4.5
±2
Bits
LSB max
LSB max
LSB max
LSB max
VIN+ − V IN–
VREF
V
VREF
−0.1 to +0.4
−0.1 to +1.5
±1
30/10
V
V
V
µA max
pF typ
2.55
±1
10/30
V
µA max
pF typ
2.4
0.8
±1
10
V min
V max
µA max
pF max
VDD = 4.75 V to 5.25 V, ISOURCE = 200 µA
VDD = 2.7 V to 3.6 V, ISOURCE = 200 µA
ISINK = 200 µA
2.8
2.4
0.4
±1
10
Straight
natural binary
V min
V min
V max
µA max
pF max
1.6 µs with a 10 MHz SCLK
16
1
100
SCLK cycles
µs max
kSPS max
Test Conditions/Comments
fIN = 30 kHz
−84 dB typ
−86 dB typ
fa = 25 kHz; fb = 35 kHz
@ −3 dB
@ −0.1 dB
VDD = 2.7 V to 3.6 V
VDD = 4.75 V to 5.25 V
When in track/hold
±1% tolerance for specified performance
When in track/hold
Typically 10 nA, VIN = 0 V or VDD
See Serial Interface section
Rev. 0 | Page 3 of 20
AD7457
Parameter
POWER REQUIREMENTS
VDD
IDD7,8
During Conversion6
Normal Mode (Static)
Normal Mode (Operational)
Power-Down
Power Dissipation
Normal Mode (Operational)
Power-Down
B Version1
Unit
2.7/5.25
V min/max
VDD = 4.75 V to 5.25 V
VDD = 2.7 V to 3.6 V
SCLK on or off
VDD = 4.75 V to 5.25 V
VDD= 2.7 V to 3.6 V
SCLK on or off
1.5
1.2
0.5
0.7
0.33
1
mA max
mA max
mA typ
mA max
mA max
µA max
VDD = 5 V
VDD = 3 V
VDD = 5 V; SCLK on or off
VDD = 3 V; SCLK on or off
3
0.9
5
3
mW max
mW max
µW max
µW max
Test Conditions/Comments
1
Temperature ranges as follows: B version: −40°C to +85°C.
See Terminology section.
3
Analog inputs with slew rates exceeding 27 V/µs (full-scale input sine wave > 3.5 MHz) within the acquisition time may cause an incorrect result to be returned by the
converter.
4
A dc input is applied to VIN– to provide a pseudo ground for VIN+.
5
The AD7457 is functional with a reference input in the range 100 mV to VDD.
6
Guaranteed by characterization.
7
See Power Consumption section.
8
Measured with a full-scale dc input.
2
Rev. 0 | Page 4 of 20
AD7457
TIMING SPECIFICATIONS1
VDD = 2.7 V to 5.25 V, fSCLK = 10 MHz, fS = 100 kSPS, VREF = 2.5 V, TA = TMIN to TMAX, unless otherwise noted.
Table 2.
Parameter
fSCLK2
tCONVERT
t2
t33
t43
t5
t6
t7
t84
tPOWER-UP5
tPOWER-DOWN
Limit at TMIN, TMAX
10
10
16 × tSCLK
1.6
10
20
40
0.4 tSCLK
0.4 tSCLK
10
10
35
1
7.4
Unit
kHz min
MHz max
Description
tSCLK = 1/fSCLK
µs max
ns min
ns max
ns max
ns min
ns min
ns min
ns min
ns max
µs max
µs min
CS rising edge to SCLK falling edge setup time
Delay from CS rising edge until SDATA three-state disabled
Data access time after SCLK falling edge
SCLK high pulse width
SCLK low pulse width
SCLK edge to data valid hold time
SCLK falling edge to SDATA three-state enabled
SCLK falling edge to SDATA three-state enabled
Power-up time from full power-down
Minimum time spent in power-down
1
The timing specifications are guaranteed by characterization. All input signals are specified with tr = tf = 5 ns (10% to 90% of VDD) and timed from a voltage level of
1.6 V. See Figure 2 and the Serial Interface section.
Mark/space ratio for the SCLK input is 40/60 to 60/40.
3
Measured with the load circuit of Figure 3 and defined as the time required for the output to cross 0.8 V or 2.4 V with VDD = 5 V, and the time required for the output to
cross 0.4 V or 2.0 V for VDD = 3 V.
4
t8 is derived from the measured time taken by the data outputs to change 0.5 V when loaded with the circuit of Figure 3. The measured number is then extrapolated
back to remove the effects of charging or discharging the 25 pF capacitor. This means that the time, t8, quoted in the timing characteristics, is the true bus relinquish
time of the part and is independent of the bus loading.
5
See Power Consumption section.
2
CONVERT
START
HOLD
TRACK
TPOWERUP
CS
SCLK
TACQUISTION
TACQUISITION
THREE-STATE
AUTOMATIC
POWER DOWN
t5
t2
t3
SDATA
TRACK
TPOWERUP
0
0
t6
t4
0
0
DB11 DB10
t8
t7
DB2
DB1
TPOWERDOWN
DB0
4 LEADING ZEROS
Figure 2. AD7457 Serial Interface Timing Diagram
Rev. 0 | Page 5 of 20
THREE-STATE
03157-0-001
POWER
UP
AD7457
ABSOLUTE MAXIMUM RATINGS
TA = 25°C, unless otherwise noted.
Table 3.
Rating
−0.3 V to +7 V
−0.3 V to VDD + 0.3 V
−0.3 V to VDD + 0.3 V
−0.3 V to +7 V
−0.3 V to VDD + 0.3 V
−0.3 V to VDD + 0.3 V
±10 mA
Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress
rating only; functional operation of the device at these or any
other conditions above those listed in the operational sections
of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.
−40°C to +85°C
−65°C to +150°C
150°C
211.5°C/W (SOT-23)
91.99°C/W (SOT-23)
IOL
1.6mA
TO
OUTPUT
PIN
1.6V
CL
25pF
215°C
220°C
IOH
200µA
03157-0-012
Parameters
VDD to GND
VIN+ to GND
VIN– to GND
Digital Input Voltage to GND
Digital Output Voltage to GND
VREF to GND
Input Current to Any Pin except
Supplies1
Operating Temperature Range
Commercial (B Version)
Storage Temperature Range
Junction Temperature
θJA Thermal Impedance
θJC Thermal Impedance
Lead Temperature, Soldering
Vapor Phase (60 s)
Infrared (15 s)
1
Transient currents of up to 100 mA do not cause SCR latch-up.
Figure 3. Load Circuit for Digital Output Timing Specifications
ESD CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the
human body and test equipment and can discharge without detection. Although this product features
proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy
electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance
degradation or loss of functionality.
Rev. 0 | Page 6 of 20
AD7457
VDD 1
SCLK 2
AD7457
8
VREF
7
VIN+
SDATA 3
6 VIN–
TOP VIEW
CS 4 (Not to Scale) 5 GND
03157-0-002
PIN CONFIGURATION AND FUNCTIONAL DESCRIPTIONS
Figure 4. 8-Lead SOT-23 Pin Configuration
Table 4. Pin Function Descriptions
Pin No.
1
Mnemonic
VDD
2
SCLK
3
SDATA
4
CS
5
GND
6
VIN–
7
8
VIN+
VREF
Description
Power Supply Input. VDD is 2.7 V to 5.25 V. This supply should be decoupled to GND with a 0.1 µF capacitor and a
10 µF tantalum capacitor.
Serial Clock. Logic input. SCLK provides the serial clock for accessing data from the part. This clock input is also
used as the clock source for the conversion process.
Serial Data. Logic output. The conversion result from the AD7457 is provided on this output as a serial data
stream. The bits are clocked out on the falling edge of the SCLK input. The data stream of the AD7457 consists of
four leading zeros followed by the 12 bits of conversion data that are provided MSB first. The output coding is
straight (natural) binary.
Chip Select. This input provides the dual function of powering up the device and initiating a conversion on the
AD7457.
Analog Ground. Ground reference point for all circuitry on the AD7457. All analog input signals and any external
reference signal should be referred to this GND voltage.
Inverting Input. This pin sets the ground reference point for the VIN+ input. Connect to ground or to a dc offset to
provide a pseudo ground.
Noninverting Analog Input.
Reference Input for the AD7457. An external reference in the range 100 mV to VDD must be applied to this input.
The specified reference input is 2.5 V. This pin should be decoupled to GND with a capacitor of at least 0.33 µF.
Rev. 0 | Page 7 of 20
AD7457
TERMINOLOGY
Signal to (Noise + Distortion) Ratio (SINAD)
The measured ratio of signal to (noise + distortion) at the
output of the ADC. The signal is the rms amplitude of the
fundamental. Noise is the sum of all nonfundamental signals up
to half the sampling frequency (fS/2), excluding dc. The ratio is
dependent on the number of quantization levels in the digitization process; the more levels, the smaller the quantization noise.
The theoretical signal to (noise + distortion) ratio for an ideal
N-bit converter with a sine wave input is given by
As a result, the second and third order terms are specified
separately. The calculation of the intermodulation distortion is
as per the THD specification, where it is the ratio of the rms
sum of the individual distortion products to the rms amplitude
of the sum of the fundamentals expressed in dB.
Signal to (Noise + Distortion) = (6.02 N + 1.76 ) dB
Aperture Jitter
The sample-to-sample variation in the effective point in time at
which the actual sample is taken.
Therefore, for a 12-bit converter, the SINAD is 74 dB.
Total Harmonic Distortion (THD)
The ratio of the rms sum of harmonics to the fundamental. For
the AD7457, it is defined as
THD (dB ) = 20 log
V22 + V32 + V42 + V52 + V62
Aperture Delay
The amount of time from the leading edge of the sampling
clock until the ADC actually takes the sample.
Full-Power Bandwidth
The full-power bandwidth of an ADC is that input frequency at
which the amplitude of the reconstructed fundamental is
reduced by 0.1 dB or 3 dB for a full-scale input.
Integral Nonlinearity (INL)
The maximum deviation from a straight line passing through
the endpoints of the ADC transfer function.
V1
where:
V1 is the rms amplitude of the fundamental.
V2, V3, V4, V5, and V6 are the rms amplitudes of the second to the
sixth harmonics.
Peak Harmonic or Spurious Noise
The ratio of the rms value of the next largest component in the
ADC output spectrum (up to fS/2 and excluding dc) to the rms
value of the fundamental. Normally, the value of this specification is determined by the largest harmonic in the spectrum, but,
for ADCs where the harmonics are buried in the noise floor, it is
a noise peak.
Intermodulation Distortion
With inputs consisting of sine waves at two frequencies, fa and
fb, any active device with nonlinearities creates distortion
products at sum and difference frequencies of mfa ± nfb, where
m, n = 0, 1, 2, 3, and so on. Intermodulation distortion terms are
those for which neither m nor n are equal to zero. For example,
the second order terms include (fa + fb) and (fa − fb), while the
third order terms include (2fa + fb), (2fa − fb), (fa + 2fb) and
(fa − 2fb).
The AD7457 is tested using the CCIF standard, where two input
frequencies near the top end of the input bandwidth are used.
In this case, the second order terms are usually distanced in
frequency from the original sine waves, while the third order
terms are usually at a frequency close to the input frequencies.
Differential Nonlinearity (DNL)
The difference between the measured and the ideal 1 LSB
change between any two adjacent codes in the ADC.
Offset Error
The deviation of the first code transition (000...000 to 000...001)
from the ideal (that is, AGND + 1 LSB).
Gain Error
The deviation of the last code transition (111...110 to 111...111)
from the ideal (that is, VREF − 1 LSB), after the offset error has
been adjusted out.
Track-and-Hold Acquisition Time
The minimum time required for the track-and-hold amplifier to
remain in track mode for its output to reach and settle to within
0.5 LSB of the applied input signal.
Power Supply Rejection Ratio (PSRR)
The ratio of the power in the ADC output at full-scale
frequency, f, to the power of a 100 mV p-p sine wave applied to
the ADC VDD supply of frequency fs. The frequency of this
input varies from 1 kHz to 1 MHz.
PSRR (dB ) = 10 log (Pf Pfs )
Pf is the power at frequency f in the ADC output; Pfs is the
power at frequency fs in the ADC output.
Rev. 0 | Page 8 of 20
AD7457
TYPICAL PERFORMANCE CHARACTERISTICS
Default Conditions: TA = 25°C, fS = 100 kSPS, fSCLK = 10 MHz, VDD = 2.7 V to 5.25 V, VREF = 2.5 V, unless otherwise noted.
1.0
75
0.8
0.6
DNL ERROR (LSB)
SINAD (dB)
VDD = 5V
VDD = 3V
70
0.4
0.2
0
–0.2
–0.4
65
10
30
20
FREQUENCY (kHz)
–0.8
–1.0
0
50
40
03157-0-017
03157-0-014
–0.6
Figure 5. SINAD vs. Analog Input Frequency for VDD = 3 V and 5 V
1024
2048
CODE
3072
4096
Figure 8. Typical DNL for the AD7457 for VDD = 5 V
1.0
0
100mV p-p SINE WAVE ON VDD
NO DECOUPLING ON VDD
–20
0.8
0.6
INL ERROR (LSB)
PSRR (dB)
–40
–60
VDD = 3V
–80
VDD = 5V
0.4
0.2
0
–0.2
–0.4
–100
–140
0
100
200 300 400 500 600 700 800
SUPPLY RIPPLE FREQUENCY (kHz)
03157-0-018
03157-0-015
–0.6
–120
–0.8
–1.0
0
900 1000
Figure 6. PSRR vs. Supply Ripple Frequency without Supply Decoupling
0
2048
CODE
–40
3072
4096
Figure 9. Typical INL for the AD7457 for VDD = 5 V
10000
8192 POINT FFT
fSAMPLE = 100kSPS
fIN = 30kHz
SINAD = 71dB
THD = –82dB
SFDR =–83dB
–20
9949
CODES
9000
8000
7000
6000
–60
5000
–80
4000
3000
–100
2000
03157-0-016
SNR (dB)
1024
–120
–140
0
30
50
FREQUENCY (kHz)
1000
27 CODES
0
2046
100
2047
24 CODES
2048
2049
2050
2051
CODES
Figure 7. Dynamic Performance for VDD = 5 V
Figure 10. Histogram of 10,000 Conversions of a DC Input
Rev. 0 | Page 9 of 20
AD7457
12
4.0
VDD = 3V
EFFECTIVE NUMBER OF BITS (LSB)
3.5
CHANGE IN DNL (LSB)
3.0
2.5
2.0
1.5
POSITIVE DNL
1.0
0.5
0
–0.5
VDD = 5V
10
9
8
7
NEGATIVE DNL
6
–1.0
0
0.5
1.0
1.5
2.0
2.5
3.0
0
3.5
VREF (V)
Figure 11. Changes in DNL vs. VREF for VDD = 5 V
4
3
2
POSITIVE INL
1
0
NEGATIVE INL
–1
–2
0
0.5
1.0
1.5
2.0
2.5
3.0
0.5
1.0
1.5
2.0
VREF (V)
2.5
Figure 13. ENOB vs. VREF for VDD =3V and 5 V
5
CHANGE IN INL (LSB)
11
3.5
VREF (V)
Figure 12. Change in INL vs. VREF for VDD = 5 V
Rev. 0 | Page 10 of 20
3.0
3.5
AD7457
THEORY OF OPERATION
CIRCUIT INFORMATION
CAPACITIVE
DAC
The AD7457 is a 12-bit, low power, single supply, successive
approximation analog-to-digital converter (ADC) with a
pseudo differential analog input. It operates with a single 2.7 V
to 5.25 V power supply and is capable of throughput rates up to
100 kSPS. It requires an external reference to be applied to the
VREF pin.
The AD7457 has an on-chip differential track-and-hold
amplifier, a successive approximation (SAR) ADC, and a serial
interface, housed in an 8-lead SOT-23 package. The serial clock
input accesses data from the part and provides the clock source
for the successive approximation ADC. The AD7457 automatically powers down after conversion, resulting in low power
consumption.
CS
B
VIN+
A
A
B
VIN–
SW1
SW2
CS
VREF
CONTROL
LOGIC
SW3
COMPARATOR
CAPACITIVE
DAC
Figure 15. ADC Conversion Phase
ADC TRANSFER FUNCTION
The output coding for the AD7457 is straight (natural) binary.
The designed code transitions occur at successive LSB values
(1 LSB, 2 LSB, and so on). The LSB size is VREF/4096. The ideal
transfer characteristics of the AD7457 are shown in Figure 16.
CONVERTER OPERATION
1LSB = VREF/4096
The AD7457 is a successive approximation ADC based around
two capacitive DACs. Figure 14 and Figure 15 show simplified
schematics of the ADC in the acquisition phase and the conversion phase, respectively. The ADC is comprised of control logic,
a SAR, and two capacitive DACs. In Figure 14 (acquisition
phase), SW3 is closed, SW1 and SW2 are in Position A, the
comparator is held in a balanced condition, and the sampling
capacitor arrays acquire the differential signal on the input.
ADC CODE
111...11
111...10
111...00
011...11
000...10
000...01
000...00
0V 1LSB
CAPACITIVE
DAC
VIN–
A
A
B
Figure 16. Ideal Transfer Characteristics
CS
B
VIN+
SW1
SW2
VREF
SW3
CS
VREF –1LSB
ANALOG INPUT
TYPICAL CONNECTION DIAGRAM
CONTROL
LOGIC
COMPARATOR
CAPACITIVE
DAC
Figure 14. ADC Acquisition Phase
When the ADC starts a conversion (Figure 15), SW3 opens, and
SW1 and SW2 move to Position B, causing the comparator to
become unbalanced. Both inputs are disconnected once the
conversion begins. The control logic and the charge redistribution DACs are used to add and subtract fixed amounts of charge
from the sampling capacitor arrays to bring the comparator
back into a balanced condition. When the comparator is
rebalanced, the conversion is complete. The control logic
generates the ADC’s output code. The output impedances of the
sources driving the VIN+ and the VIN– pins must be matched;
otherwise the two inputs have different settling times, resulting
in errors.
Figure 17 shows a typical connection diagram for the AD7457.
In this setup, the GND pin is connected to the analog ground
plane of the system. The VREF pin is connected to the AD780, a
2.5 V decoupled reference source. The signal source is connected to the VIN+ analog input via a unity gain buffer. A dc
voltage is connected to the VIN– pin to provide a pseudo ground
for the VIN+ input. The VDD pin should be decoupled to AGND
with a 10 µF tantalum capacitor in parallel with a 0.1 µF
ceramic capacitor. The reference pin should be decoupled to
AGND with a capacitor of at least 0.33 µF. The conversion result
is output in a 16-bit word with four leading zeros followed by
the MSB of the 12-bit result.
Rev. 0 | Page 11 of 20
AD7457
10µF
0.1µF
+2.7V TO +5.25V
SUPPLY
ANALOG INPUT STRUCTURE
ANALOG INPUT
Figure 19 shows the equivalent circuit of the analog input
structure of the AD7457. 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, which causes 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 19, are typically
4 pF and can be attributed primarily 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,
which typically have a capacitance of 16 pF.
The AD7457 has a pseudo differential analog input. The VIN+
input is coupled to the signal source and should have an
amplitude of VREF p-p to make use of the full dynamic range of
the part. A dc input is applied to the VIN–. The voltage applied to
this input provides an offset from ground or a pseudo ground
for the VIN+ input. Ensure that (VIN− + VIN+) is less than or equal
to VDD to avoid exceeding the maximum ratings of the ADC.
The main benefit of pseudo differential inputs is that they
separate the analog input signal ground from the ADC’s ground,
allowing dc common-mode voltages to be canceled.
For ac applications, removing high frequency components from
the analog input signal through the use of an RC low pass filter
on the relevant analog input pins is recommended. In applications where harmonic distortion and the signal-to-noise ratio
are critical, the analog input should be driven from a low
impedance source. Large source impedances can significantly
affect the ac performance of the ADC, which may necessitate
the use of an input buffer amplifier. The choice of the op amp is
a function of the particular application.
SERIAL
INTERFACE
VDD
AD7457
VREF
P-TO-P
VIN+
SCLK
µC/µP
SDATA
CS
VIN–
GND
VREF
0.33µF
2.5V
AD780
03157-0-006
DC INPUT
VOLTAGE
Figure 17. Typical Connection Diagram
VDD
Because the ADC operates from a single supply, it is necessary
to level shift ground-based bipolar signals to comply with the
input requirements. An op amp (for example, the AD8021) can
be configured to rescale and level shift a ground-based (bipolar)
signal, so that it is compatible with the input range of the
AD7457. See Figure 18.
D
C1
+1.25V
0V
–1.25V
R
VIN
R1
C2
VDD
2.5V
1.25V
0V
D
VIN–
C1
VIN+
3R
C2
D
When a conversion takes place, the pseudo ground corresponds
to 0 and the maximum analog input corresponds to 4096.
R
R1
VIN+
D
AD7457
R
0.33µF
VIN–
VREF
EXTERNAL
VREF (2.5V)
03157-0-007
Figure 19. Equivalent Analog Input Circuit. Conversion Phase—Switches
Open; Track Phase—Switches Closed
Figure 18. Op Amp Configuration to Level Shift a Bipolar Input Signal
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 20 shows a graph of the THD versus analog input signal
frequency for different source impedances.
Rev. 0 | Page 12 of 20
AD7457
–50
errors in the AD7457 transfer function. A capacitor of at least
0.33 µF should be placed on the VREF pin. Suitable reference
sources for the AD7457 include the AD780 and the ADR421.
Figure 22 shows a typical connection diagram for the VREF pin.
TA = 25°C
–60
200Ω
VDD
100Ω
NC
10
VDD
62Ω
10Ω
20
30
INPUT FREQUENCY (kHz)
03157-0-009
–90
AD7457*
AD780
–80
40
0.1µF
1
OPSEL 8
NC
7
NC
2.5V
2 VIN
10µF
0.1µF
3 TEMP
VOUT 6
4 GND
TRIM 5
NC
VREF
0.33µF
NC = NO CONNECT
50
* ADDITIONAL PINS OMITTED FOR CLARITY
03157-0-011
THD (dB)
–70
Figure 20. THD vs. Analog Input Frequency for Various Source Impedances
Figure 22. Typical VREF Connection Diagram for VDD = 5 V
Figure 21 shows a graph of THD versus analog input frequency
for various supply voltages, while sampling at 100 kSPS with an
SCLK of 10 MHz. In this case, the source impedance is 10 Ω.
–50
TA = 25°C
–55
–60
THD (dBs)
–70
VDD = 3.6V
VDD = 2.7V
–80
03157-0-010
VDD = 4.75V
–85
–90
10
VDD = 5.25V
20
30
INPUT FREQUENCY (kHz)
40
Figure 2 shows a detailed timing diagram of the serial interface
of the AD7457. The serial clock provides the conversion clock
and also controls the transfer of data from the device during
conversion.
The falling edge of CS powers up the AD7457 and also puts the
track-and-hold into track. The power-up time is 1 µs minimum
and, in this time, the device also acquires the analog input
signal. CS must remain low for the duration of power-up. The
rising edge of CS initiates the conversion process, puts the
track-and-hold into hold mode, and takes the serial data bus out
of three-state. The conversion requires 16 SCLK cycles to
complete.
–65
–75
SERIAL INTERFACE
50
Figure 21. THD vs. Analog Input Frequency for Various Supply Voltages
DIGITAL INPUTS
The digital inputs applied to the AD7457 are not limited by the
maximum ratings that limit the analog inputs. Instead the
digital inputs applied, that is, CS and SCLK, can go to 7 V and
are not restricted by the VDD + 0.3 V limits as on the analog
input.
The main advantage of the inputs not being restricted to the
VDD + 0.3 V limit is that power supply sequencing issues are
avoided. If CS or SCLK are applied before VDD, 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 VDD.
REFERENCE SECTION
An external source is required to supply the reference to the
AD7457. This reference input can range from 100 mV to VDD.
The specified reference is 2.50 V for the power supply range
2.70 V to 5.25 V. Errors in the reference source result in gain
On the sixteenth SCLK falling edge, after the time t8, the serial
data bus goes back into three-state and the device automatically
enters full power-down. It remains in power-down until the
next falling edge of CS. For specified performance, the throughput rate should not exceed 100 kSPS, which means that there
should be no less than 10 µs between consecutive CS falling
edges.
The conversion result from the AD7457 is provided on the
SDATA output as a serial data stream. The bits are clocked out
on the falling edge of the SCLK input. The data stream of the
AD7457 consists of four leading zeros, followed by the 12 bits of
conversion data that are provided MSB first. The output coding
is straight (natural) binary.
Sixteen serial clock cycles are, therefore, required to perform a
conversion and to access data from the AD7457. A rising edge
of CS provides the first leading zero to be read in by the
microcontroller or DSP. The remaining data is then clocked out
on the subsequent SCLK falling edges beginning with the
second leading zero. Thus, the first falling clock edge on the
serial clock after CS has gone high provides the second leading
zero. The final bit in the data transfer, before the device goes
Rev. 0 | Page 13 of 20
AD7457
2.5
into power-down, is valid on the sixteenth falling edge of SCLK,
having been clocked out on the previous (fifteenth) falling edge.
2.0
POWER (mW)
The conversion time of the device is determined by the serial
clock frequency. The faster the SCLK frequency, the shorter the
conversion time. Therefore, as the clock frequency used is
increased, the ADC is dissipating power for a shorter period of
time (during conversion) and it remains in power-down for a
longer percentage of the cycle time or throughput rate. This can
be seen in Figure 23, which shows typical IDD versus SCLK
frequency for VDD of 3 V and 5 V, when operating the device at
the maximum throughput of 100 kSPS.
2.5
VDD = 5V
1.0
0.5
VDD = 3V
0
POWER CONSUMPTION
The AD7457 automatically enters power-down at the end of
each conversion. When in the power-down mode, all analog
circuitry is powered down and the current consumption is 1 µA.
To achieve the specified power consumption (which is the
lowest), there are a few things the user should keep in mind.
1.5
03157-0-024
In applications with a slow SCLK, it is possible to read in data
on each SCLK rising edge. In this case, the first falling edge of
SCLK after the CS rising edge clocks out the second leading
zero and can be read in on the following rising edge. If the first
SCLK edge after the CS rising edge is a falling edge, the first
leading zero that was clocked out when CS went high is missed,
unless it was not read on the first SCLK falling edge. The
fifteenth falling edge of SCLK clocks out the last bit of data,
which can be read in by the following rising SCLK edge.
TA = 25°C
0
20
40
60
THROUGHPUT (kSPS)
80
100
Figure 24. Power vs. Throughput Rate for SCLK = 10 MHz for VDD = 3 V and 5 V
MICROPROCESSOR INTERFACING
The serial interface of the AD7457 allows the part to be connected to a range of different microprocessors. This section
explains how to interface the AD7457 with the ADSP-218x
serial interface.
AD7457 to ADSP-218x
The ADSP-218x family of DSPs can be interfaced directly to the
AD7457 without any glue logic. The serial clock for the ADC is
provided by the DSP. SDATA from the ADC is connected to the
data receive (DR) input of the serial port and CS can be
controlled by a flag (FL0). The connection diagram is shown in
Figure 25.
AD7457*
TA = 25°C
2.0
ADSP-21xx*
SCLK
SCLK
SDATA
DR0
SPORT0
RFS
CS
1.0
*ADDITIONAL PINS OMITTED FOR CLARITY
VDD = 5V
VDD = 3V
SPORT1
Figure 25. AD7457 to ADSP-218x
03157-0-023
0.5
0
0
FL0
03157-0-025
IDD (mA)
1.5
2
4
6
SCLK Frequency (MHz)
8
10
Figure 23. IDD vs. SCLK Frequency for VDD = 3 V and 5 V when Operating at 100
kSPS
SPORT0 must be enabled to receive the conversion data and to
provide the SCLK, while SPORT1 must be configured for flags,
and so on.
SPORT0 is configured by setting the bits in its control register
as listed in Table 5.
Figure 24 shows typical power consumption versus throughput
rate for the maximum SCLK frequency of 10 MHz. In this case,
the conversion time is the same for all throughputs, because the
SCLK frequency is fixed. As the throughput rate decreases, the
average power consumption decreases, because the ADC spends
more time in power-down.
Rev. 0 | Page 14 of 20
AD7457
Table 5. SPORT0 Configuration
Bit
ISCLK
SLEN
RFSR
Setting
1
1111
0
TFSR
IRFS
Don’t care
0
ITFS
RFSW
TFSW
INVRFS
INVTFS
Don’t care
1
Don’t care
0
Don’t care
Comment/Description
Serial clock generated internally
16 bits of conversion data
Receive frame sync required every
word
Not used
RFS is set to be an input and is
generated externally
Not used
Alternate receive framing
Not used
RFS is active high
Not used
The flag to generate the CS signal is generated by SPORT1. It is
connected to both the ADC and the RFS input of SPORT0 to
provide the frame sync signal for the DSP.
Rev. 0 | Page 15 of 20
AD7457
APPLICATION HINTS
GROUNDING AND LAYOUT
The printed circuit board that houses the AD7457 should be
designed so that the analog and digital sections are separated
and confined to certain areas of the board. This facilitates the
use of ground planes that can be easily separated. A minimum
etch technique is generally best for ground planes, because it
gives the best shielding. Digital and analog ground planes
should be joined in only one place, and the connection should
be a star ground point established as close as possible to the
GND pin on the AD7457.
Avoid running digital lines under the device, because this
couples noise onto the die. The analog ground plane should be
allowed to run under the AD7457 to avoid noise coupling. The
power supply lines to the AD7457 should use as large a trace as
possible to provide low impedance paths and reduce the effects
of glitches on the power supply line. Fast switching signals such
as clocks should be shielded with digital ground to avoid
radiating noise to other sections of the board, and clock signals
should never run near the analog inputs. Avoid crossover of
digital and analog signals. Traces on opposite sides of the board
should run at right angles to each other. This reduces the effects
of feed through the board. A micro strip technique is by far the
best, but is not always possible with a double-sided board.
In this technique, the component side of the board is dedicated
to ground planes, while signals are placed on the solder side.
Good decoupling is also important. All analog supplies should
be decoupled with 10 µF tantalum capacitors in parallel with
0.1 µF capacitors to GND. To achieve the best from these
decoupling components, place them as close as possible to the
device.
Rev. 0 | Page 16 of 20
AD7457
OUTLINE DIMENSIONS
2.90 BSC
8
7
6
5
1
2
3
4
1.60 BSC
2.80 BSC
PIN 1
0.65 BSC
1.30
1.15
0.90
1.95
BSC
1.45 MAX
0.15 MAX
0.38
0.22
0.22
0.08
SEATING
PLANE
8°
4°
0°
0.60
0.45
0.30
COMPLIANT TO JEDEC STANDARDS MO-178BA
Figure 26. 8-Lead Small Outline Transistor Package [SOT-23] (RT-8)—Dimensions shown in millimeters
ORDERING GUIDE
Model
AD7457BRT-R2
AD7457BRT-REEL7
1
Temperature Range
–40°C to +85°C
–40°C to +85°C
Linearity Error (LSB)1
±1
±1
Linearity error here refers to integral nonlinearity error.
Rev. 0 | Page 17 of 20
Package Description
8-Lead SOT-23
8-Lead SOT-23
Package Option
RT-8
RT-8
Branding
COD
COD
AD7457
NOTES
Rev. 0 | Page 18 of 20
AD7457
NOTES
Rev. 0 | Page 19 of 20
AD7457
NOTES
© 2003 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
C03157–0–10/03(0)
Rev. 0 | Page 20 of 20