Burr-Brown ADS7835EB 12-bit, high-speed, low power sampling analog-to-digital converter Datasheet

ADS7835
®
For most current data sheet and other product
information, visit www.burr-brown.com
12-Bit, High-Speed, Low Power Sampling
ANALOG-TO-DIGITAL CONVERTER
FEATURES
DESCRIPTION
● 500kHz THROUGHPUT RATE
The ADS7835 is a 12-bit, sampling analog-to-digital converter (A/D) complete with sample-and-hold
(S/H), internal 2.5V reference, and synchronous
serial interface. Typical power dissipation is 17.5mW
at a 500kHz throughput rate. The device can be
placed into a power-down mode which reduces dissipation to just 2.5mW. The input range is –V REF to
+VREF, and the internal reference can be overdriven
by an external voltage.
● 2.5V INTERNAL REFERENCE
● LOW POWER: 17.5mW
● SINGLE SUPPLY +5V OPERATION
● SERIAL INTERFACE
● GUARANTEED NO MISSING CODES
● MSOP-8
● ±VREF INPUT RANGE
Low power, small size, and high speed make the
ADS7835 ideal for battery-operated systems such
as wireless communication devices, portable multichannel data loggers, and spectrum analyzers. The
serial interface also provides low cost isolation for
remote data acquisition. The ADS7835 is available in an MSOP-8 package and is guaranteed over
the –40°C to +85°C temperature range.
APPLICATIONS
● BATTERY-OPERATED SYSTEMS
● DIGITAL SIGNAL PROCESSING
● HIGH-SPEED DATA ACQUISITION
● WIRELESS COMMUNICATION SYSTEMS
CLK
SAR
CONV
±2.5V
Input
2kΩ
CDAC
2kΩ
Serial
Interface
Comparator
S/H Amp
Internal
+2.5V Ref
Buffer
VREF
DATA
10kΩ ±30%
International Airport Industrial Park • Mailing Address: PO Box 11400, Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd., Tucson, AZ 85706 • Tel: (520) 746-1111
Twx: 910-952-1111 • Internet: http://www.burr-brown.com/ • Cable: BBRCORP • Telex: 066-6491 • FAX: (520) 889-1510 • Immediate Product Info: (800) 548-6132
®
©
1998 Burr-Brown Corporation
PDS-1478B
1
Printed in U.S.A.May, 2000
ADS7835
SPECIFICATIONS
At TA = –40°C to +85°C, +VCC = +5V, fSAMPLE = 500kHz, fCLK = 16 • fSAMPLE, and internal +2.5V reference, unless otherwise specified.
ADS7835E
PARAMETER
CONDITIONS
MIN
RESOLUTION
TYP
ADS7835EB
MAX
MIN
Input Capacitance
Input Resistance
±2.5V with the 2.5V
Internal Reference
–VREF
SYSTEM PERFORMANCE
No Missing Codes
Integral Linearity
Differential Linearity
Bipolar Offset Error
Positive Fulll-Scale Error(3)
Negative Full-Scale Error(3)
Noise
Power Supply Rejection Ratio
At
–40°C
At
–40°C
±10
±20
±35
±20
±35
✻
✻
✻
µs
µs
kHz
ns
ps
ns
IOUT = 0
Static Load
4.75V ≤ VCC ≤ 5.25V
5Vp-p
5Vp-p
5Vp-p
5Vp-p
at
at
at
at
10kHz
10kHz
10kHz
10kHz
68
72
72
–78
70
78
2.475
2.50
✻
2.3
IIH ≤ +5µA
IIL ≤ +5µA
IOH = –500µA
IOL = 500µA
Specified Performance
fSAMPLE = 500kHz
Power-Down
70
75
✻
–82
72
82
2.48
✻
–72
2.525
50
2.9
✻
10
✻
CMOS
✻
3.0
–0.3
3.5
+VCC + 0.3
0.8
✻
✻
✻
5.25
3.5
0.5
17.5
2.5
Power Dissipation
Power-Down
–40
V
µA
mV
✻
V
kΩ
✻
✻
V
V
V
V
✻
✻
✻
✻
✻
✻
✻
30
+85
dB
dB
dB
dB
2.52
✻
✻
0.4
Binary Two’s Complement
4.75
–75
✻
0.2
To Internal Reference Voltage
±1
±1
±5
±12
±25
±12
±25
✻
✻
500
=
=
=
=
pF
kΩ
✻
✻
±7
200
0.3
V
Bits
LSB(2)
LSB
LSB
LSB
LSB
LSB
LSB
µVrms
LSB
±0.5
±0.5
±1
±7
5
30
375
REFERENCE OUTPUT
Voltage
Source Current(5)
Line Regulation
TEMPERATURE RANGE
Specified Performance
±2
1.625
0.350
VIN
VIN
VIN
VIN
POWER SUPPLY REQUIREMENT
+VCC
Quiescent Current
±12
Worst-Case ∆, +VCC = 5V ±5%
✻
✻
±1
±0.8
±2
±12
25°C
to +85°C
25°C
to +85°C
DYNAMIC CHARACTERISTICS
Signal-to-Noise Ratio
Total Harmonic Distortion(4)
Signal-to-(Noise+Distortion)
Spurious Free Dynamic Range
DIGITAL INPUT/OUTPUT
Logic Family
Logic Levels:
VIH
VIL
VOH
VOL
Data Format
✻
UNITS
Bits
✻
✻
12
SAMPLING DYNAMICS
Conversion Time
Acquisition Time
Throughput Rate
Aperture Delay
Aperture Jitter
Step Response
REFERENCE INPUT
Range
Resistance(6)
+VREF
25
2
During Conversion (CONV = LOW)
MAX
✻
12
ANALOG INPUT(1)
Input Voltage Range
TYP
✻
✻
✻
V
mA
mA
mW
mW
°C
✻ Specifications same as ADS7835E.
NOTES: (1) Ideal input span, does not include gain or offset error. (2) LSB means Least Significant Bit, with VREF equal to +2.5V, one LSB is 1.22mV. (3) Measured
relative to an ideal positive full scale of 2.499V for positive full-scale error. Measured relative to an ideal negative full scale of –2.499V for negative full-scale error.
(4) Calculated on the first nine harmonics of the input frequency. (5) If the internal reference is required to source current to an external load, the reference voltage
will change due to the internal 10kΩ resistor. (6) Can vary ±30%.
The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN
assumes no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject
to change without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not
authorize or warrant any BURR-BROWN product for use in life support devices and/or systems.
®
ADS7835
2
ABSOLUTE MAXIMUM RATINGS(1)
ELECTROSTATIC
DISCHARGE SENSITIVITY
+VCC to GND ............................................................................ –0.3V to 6V
Analog Inputs to GND ............................................................. –5.3 to +5.3
Digital Inputs to GND ............................................... –0.3V to (VCC + 0.3V)
Power Dissipation .......................................................................... 325mW
Maximum Junction Temperature ................................................... +150°C
Operating Temperature Range ......................................... –40°C to +85°C
Storage Temperature Range .......................................... –65°C to +150°C
Lead Temperature (soldering, 10s) ............................................... +300°C
Electrostatic discharge can cause damage ranging from performance degradation to complete device failure. Burr-Brown
Corporation recommends that all integrated circuits be
handled and stored using appropriate ESD protection methods.
NOTE: (1) Stresses above those listed under “Absolute Maximum Ratings” may
cause permanent damage to the device. Exposure to absolute maximum conditions for extended periods may affect device reliability.
ESD damage can range from subtle performance degradation to complete device failure. Precision integrated circuits
may be more susceptible to damage because very small
parametric changes could cause the device not to meet
published specifications.
PIN CONFIGURATION
Top View
8
+VCC
7
CLK
3
6
DATA
4
5
CONV
VREF
1
AIN
2
GND
GND
ADS7835
MSOP-8
PIN ASSIGNMENTS
PIN
NAME
1
VREF
DESCRIPTION
Reference Output. Decouple to ground with a 0.1µF ceramic capacitor and a 2.2µF tantalum capacitor.
±2.5V Input
2
AIN
3
GND
Ground
4
GND
Ground
5
CONV
Convert Input. Controls the sample/hold mode, start of conversion, start of serial data transfer, type of serial transfer, and powerdown mode. See the Digital Interface section for more information.
6
DATA
Serial Data Output. The 12-bit conversion result is serially transmitted most significant bit first with each bit valid on the rising edge
of CLK. By properly controlling the CONV input, it is possibly to have the data transmitted least significant bit first. See the Digital
Interface section for more information.
7
CLK
Clock Input. Synchronizes the serial data transfer and determines conversion speed.
8
+VCC
Power Supply. Decouple to ground with a 0.1µF ceramic capacitor and a 10µF tantalum capacitor.
PACKAGE/ORDERING INFORMATION
PRODUCT
ADS7835E
"
MAXIMUM
INTEGRAL
LINEARITY
ERROR
(LSB)
MAXIMUM
DIFFERENTIAL
LINEARITY
ERROR
(LSB)
±2
"
PACKAGE
PACKAGE
DRAWING
NUMBER(1)
SPECIFICATION
TEMPERATURE
RANGE
PACKAGE
MARKING(2)
ORDERING
NUMBER(3)
TRANSPORT
MEDIA
N/S(4)
MSOP-8
337
–40°C to +85°C
B35
"
"
"
"
"
B35
ADS7835E/250
ADS7835E/2K5
ADS7835EB/250
ADS7835EB/2K5
ADS7835P
ADS7835PB
Tape and Reel
Tape and Reel
Tape and Reel
Tape and Reel
Rails
Rails
ADS7835EB
±1
±1
MSOP-8
337
–40°C to +85°C
"
"
"
"
"
"
"
N/S(4)
±1
Plastic DIP-8
006
–40°C to +85°C
"
"
"
ADS7835P
ADS7835PB
ADS7835P
ADS7835PB
±2
±1
NOTE: (1) For detail drawing and dimension table, please see end of data sheet or Package Drawing File on Web. (2) Performance Grade information is marked
on the reel. (3) Models with a slash(/) are available only in Tape and reel in quantities indicated (e.g. /250 indicates 250 units per reel, /2K5 indicates 2500 devices
per reel). Ordering 2500 pieces of ”ADS7835E/2K5“ will get a single 2500-piece Tape and Reel. For detailed Tape and Reel mechanical information, refer to the
www.burr-brown.com web site under Applications and Tape and Reel Orientation and Dimensions. (4) N/S = Not Specified, typical only. However, 12-Bits no missing
codes is guaranteed over temperature.
®
3
ADS7835
TYPICAL PERFORMANCE CURVES
At TA = +25°C, +VCC = +5V, fSAMPLE = 500kHz, fCLK = 16 • fSAMPLE, and internal +2.5V reference, unless otherwise specified.
CHANGE IN BIPOLAR OFFSET ERROR
vs TEMPERATURE
CHANGE IN FULL-SCALE ERRORS
vs TEMPERATURE
0.6
1.0
Negative Full-Scale Error
0.4
Delta from +25°C (LSB)
Delta from +25°C (LSB)
0.0
–1.0
–2.0
Positive Full-Scale Error
–3.0
–4.0
0.2
0.0
–0.2
–0.4
–0.6
–0.8
–5.0
–40
–1.0
–20
0
20
60
40
80
–40
100
–20
0
Temperature (°C)
40
60
80
100
SUPPLY CURRENT vs TEMPERATURE
470
3.8
460
3.7
450
3.6
Supply Current (mA)
Power-Down Supply Current (µA)
POWER-DOWN SUPPLY CURRENT
vs TEMPERATURE
440
430
420
410
400
fSAMPLE = 500kHz
3.5
3.4
3.3
3.2
fSAMPLE = 125kHz
3.1
390
–40
–20
0
20
40
60
80
3.0
–40
100
–20
0
Temperature (°C)
40
60
80
100
CHANGE IN INTEGRAL and DIFFERENTIAL LINEARITY
vs SAMPLE RATE
0.5
3.9
0.4
Delta from fSAMPLE = 500kHz (LSB)
4.0
3.8
3.7
3.6
3.5
3.4
3.3
3.2
3.1
3.0
100
20
Temperature (°C)
SUPPLY CURRENT vs SAMPLE RATE
Supply Current (mA)
20
Temperature (°C)
0.3
Change in Integral
Linearity (LSB)
0.2
0.1
0.0
–0.1
–0.2
Change in Differential
Linearity (LSB)
–0.3
–0.4
–0.5
200
300
400
500
600
0
700
®
ADS7835
100
200
300
400
Sample Rate (kHz)
Sample Rate (kHz)
4
500
600
700
TYPICAL PERFORMANCE CURVES (Cont.)
At TA = +25°C, +VCC = +5V, fSAMPLE = 500kHz, fCLK = 16 • fSAMPLE, and internal +2.5V reference, unless otherwise specified.
PEAK-TO-PEAK NOISE
vs EXTERNAL REFERENCE VOLTAGE
CHANGE END-POINT ERRORS
vs EXTERNAL REFERENCE VOLTAGE
0.90
3.0
0.85
Peak-to-Peak Noise (LSB)
Delta from VREF = 2.5V (LSB)
4.0
Negative Full-Scale Error
2.0
1.0
0.0
–1.0
–2.0
Positive Full-Scale Error
and Bipolar Offset Error
–3.0
0.80
0.75
0/70
0.65
0.60
0.55
–4.0
0.50
–5.0
2.2
2.4
2.6
2.8
2
3.0
2.1
External Reference Voltage (V)
30
0
25
–20
20
–40
Amplitude (dB)
Power Supply Rejection Ratio (mV/V)
2.3
2.4
2.5
FREQUENCY SPECTRUM
(4096 Point FFT; fIN = 977Hz, –0.2dB)
POWER SUPPLY REJECTION RATIO
vs POWER SUPPLY RIPPLE FREQUENCY
15
10
–60
–80
–100
5
–120
0
1
10
100
10k
1k
100k
0
1M
50
100
150
200
Power Supply Ripple Frequency (Hz)
Frequency (kHz)
FREQUENCY SPECTRUM
(4096 Point FFT; fIN = 9.77kHz, –0.2dB)
FREQUENCY SPECTRUM
(4096 Point FFT; fIN = 99.7kHz, –0.2dB)
0
0
–20
–20
–40
–40
Amplitude (dB)
Amplitude (dB)
2.2
External Reference Voltage (V)
–60
–80
–100
250
–60
–80
–100
–120
–120
0
50
100
150
200
250
0
Frequency (kHz)
50
100
150
200
250
Frequency (kHz)
®
5
ADS7835
TYPICAL PERFORMANCE CURVES (Cont.)
At TA = +25°C, +VCC = +5V, fSAMPLE = 500kHz, fCLK = 16 • fSAMPLE, and internal +2.5V reference, unless otherwise specified.
SPURIOUS FREE DYNAMIC RANGE and
TOTAL HARMONIC DISTORTION
vs INPUT FREQUENCY
SIGNAL-TO-NOISE and
SIGNAL-TO-(NOISE + DISTORTION)
vs INPUT FREQUENCY
75
90
SNR
85
SFDR and THD (dB)
SNR and SINAD (dB)
73
71
SINAD
69
67
SFDR
80
THD✻
75
70
65
✻ First
nine harmonics
of the input frequency
63
65
1
10
100
1000
1
10
Input Frequency (kHz)
SFDR and THD Deltas from +25°C (dB)
fIN = 10kHz, –0.2dB
0.3
0.2
SNR
0.0
–0.1
–0.2
SINAD
–0.3
–0.4
–0.5
–40
–20
0
20
40
60
80
100
2.0
fIN = 10kHz, –0.2dB
1.5
1.0
0.5
THD∗
0.0
–0.5
–1.0
SFDR
∗First nine harmonics
of the input frequency
–1.5
–2.0
–40
–20
0
600
700
Temperature (°C)
CHANGE IN BIPOLAR OFFSET ERROR
vs SAMPLE RATE
1.0
0.8
Delta from 500kHz (mA)
SNR and SINAD Deltas from +25°C (dB)
0.5
0.1
0.6
0.4
0.2
0.0
–0.2
0
100
200
300
400
Sample Rate (kHz)
®
ADS7835
1000
CHANGE IN SPURIOUS FREE DYNAMIC RANGE
and TOTAL HARMONIC DISTORTION
vs TEMPERATURE
CHANGE IN SIGNAL-TO-NOISE and
SIGNAL-TO-(NOISE+DISTORTION)
vs TEMPERATURE
0.4
100
Input Frequency (kHz)
6
500
20
40
Temperature (°C)
60
80
100
THEORY OF OPERATION
the device is in the sample or hold mode. When sampling,
the input has a 4kΩ input impedance to the reference. The
source of the analog input voltage must be able to charge the
input impedance (typically 25pF || 1kΩ) to a 12-bit settling
level within the same period. This can be as little as 350ns
in some operating modes. When the converter is in the hold
mode, the input impedance switches to approximately 2kΩ
to ground.
The ADS7835 is a high speed Successive Approximation
Register (SAR) analog-to-digital converter (A/D) with an
internal 2.5V bandgap reference. The architecture is based
on capacitive redistribution which inherently includes a S/H
function. The converter is fabricated on a 0.6µ CMOS
process. See Figure 1 for the basic operating circuit for the
ADS7835.
Care must be taken regarding the input voltage on the AIN
pin. The input signal should remain within –5.3V and +5.3V
(with a 5V supply) to avoid damaging the converter.
The ADS7835 requires an external clock to run the conversion process. This clock can vary between 200kHz (12.5kHz
throughput) and 8MHz (500kHz throughput). The duty cycle
of the clock is unimportant as long as the minimum HIGH
and LOW times are at least 50ns and the clock period is at
least 125ns. The minimum clock frequency is set by the
leakage on the capacitors internal to the ADS7835.
REFERENCE
The reference voltage on the VREF pin directly sets the fullscale range of the analog input. The ADS7835 can operate
with a reference in the range of 2.3V to 2.9V, for a full-scale
range of ±2.3V to ±2.9V.
The analog input to the ADS7835 is single-ended. The
ADS7835 provides a true bipolar input where the input will
swing below ground. When using the internal 2.5V reference the input range is ±2.5V (within ±20mV for the low
grade and ±12mV for the high grade). When using an
external reference the input range is –VREF to +VREF. The
ADS7835 will accept an external reference with a range of
2.3V to 2.9V.
The voltage at the VREF pin is internally buffered and this
buffer drives the CDAC portion of the converter. This is
important because the buffer greatly reduces the dynamic
load placed on the reference source. However, the voltage at
VREF will still contain some noise and glitches from the SAR
conversion process. These can be reduced by carefully
bypassing the VREF pin to ground as outlined in the sections
that follow.
The digital result of the conversion is provided in a serial
manner, synchronous to the CLK input. The provided result
is Most Significant Bit (MSB) first and represents the result
of the conversion currently in progress—there is no pipeline
delay. By properly controlling the CONV and CLK inputs,
it is possible to obtain the digital result Least Significant Bit
(LSB) first.
INTERNAL REFERENCE
The ADS7835 contains an on-board 2.5V reference, resulting in a –2.5V to +2.5V input range on the analog input. The
Specification table gives the various specifications for the
internal reference. This reference can be used to supply a
small amount of source current to an external load, but the
load should be static. Due to the internal 10kΩ resistor, a
dynamic load will cause variations in the reference voltage,
and will dramatically affect the conversion result. Note that
even a static load will reduce the internal reference voltage
seen at the buffer input. The amount of reduction depends on
the load and the actual value of the internal “10kΩ” resistor.
The value of this resistor can vary by ±30%.
ANALOG INPUT
The analog input (pin 2) of the ADS7835 is connected to a
2kΩ x 2kΩ voltage divider. This divider allows the ADS7835
to accept bipolar inputs while operating from a single 5V
supply. The divider is connected to the output buffer of the
internal +2.5V supply. When the input is at +full-scale
(+2.5V), the voltage at the input to the CDAC (Capacitive
Digital-to-Analog Converter) is also +2.5V resulting in
negligible input current. When the input is at –full-scale
(–2.5V), the voltage at the input of the CDAC is 0V
resulting in 1.25mA of current being sourced out of the
input pin. It is recommended that a buffer be placed
between the analog input signal and the input of the ADS7835.
The VREF pin should be bypassed with a 0.1µF capacitor
placed as close as possible to the ADS7835 package. In
addition, a 2.2µF tantalum capacitor should be used in
parallel with the ceramic capacitor. Placement of this capacitor, while not critical to performance, should be placed
as close to the package as possible.
The input impedance of the ADS7835 depends on whether
+5V
2.2µF
+
ADS7835
0.1µF
±2.5V
Analog Input
0.1µF
+
+VCC
8
CLK
7
Serial Clock
DATA
6
Serial Data
5
Convert Start
1
VREF
2
AIN
3
GND
4
GND
CONV
10µF
from
Microcontroller
or DSP
FIGURE 1. Basic Operation of the ADS7835.
®
7
ADS7835
the CLK may be kept LOW or HIGH.
EXTERNAL REFERENCE
The asynchronous nature of CONV to CLK raises some
interesting possibilities, but also some design considerations. Figure 3 shows that CONV has timing restraints in
relation to CLK (tCKCH and tCKCS). However, if these times
are violated (which could happen if CONV is completely
asynchronous to CLK), the converter will perform a conversion correctly, but the exact timing of the conversion is
indeterminate. Since the setup and hold time between CONV
and CLK has been violated in this example, the start of
conversion could vary by one clock cycle. (Note that the
start of conversion can be detected by using a pull-up
resistor on DATA. When DATA drops out of high impedance and goes LOW, the conversion has started and that
The internal reference is connected to the VREF pin and to the
internal buffer via a 10kΩ series resistor. Thus, the reference
voltage can easily be overdriven by an external reference
voltage. The voltage range for the external voltage is 2.3V
to 2.9V, corresponding to an analog input range of 2.3V to
2.9V in both cases.
While the external reference will not source significant
current into the VREF pin, it does have to drive the 10kΩ
series resistor that is terminated into the 2.5V internal
reference (the exact value of the resistor will vary up to
±30% from part to part). In addition, the VREF pin should
still be bypassed to ground with at least a 0.1µF ceramic
capacitor (placed as close to the ADS7835 as possible). The
reference will have to be stable with this capacitive load.
Depending on the particular reference and A/D conversion
speed, additional bypass capacitance may be required, such
as the 2.2µF tantalum capacitor shown in Figure 1.
Reasons for choosing an external reference over the internal
reference vary, but there are two main reasons. One is to
achieve a given input range. The other is to provide greater
stability over temperature. (The internal reference is typically 20ppm/°C which translates into a full-scale drift of
roughly one output code for every 12°C. This does not take
into account other sources of full-scale drift.) If greater
stability over temperature is needed, then an external reference with lower temperature drift will be required.
DIGITAL INTERFACE
Figure 2 shows the serial data timing and Figure 3 shows the
basic conversion timing for the ADS7835. The specific
timing numbers are listed in Table I. There are several
important items in Figure 3 which give the converter additional capabilities over typical 8-pin converters. First, the
transition from sample mode to hold mode is synchronous to
the falling edge of CONV and is not dependent on CLK.
Second, the CLK input is not required to be continuous
during the sample mode. After the conversion is complete,
SYMBOL
DESCRIPTION
MIN
tACQ
Acquisition Time
350
ns
tCONV
Conversion Time
1.625
µs
tCKP
Clock Period
125
tCKL
Clock LOW
50
tCKH
Clock HIGH
50
tCKDH
Clock Falling to Current Data
Bit No Longer Valid
5
tCKDS
Clock Falling to Next Data Valid
tCVL
CONV LOW
TYP
MAX
5000
UNITS
ns
ns
ns
15
30
ns
50
40
ns
ns
tCVH
CONV HIGH
40
ns
tCKCH
CONV Hold after Clock Falls(1)
10
ns
tCKCS
CONV Setup to Clock Falling(1)
10
tCKDE
Clock Falling to DATA Enabled
20
50
ns
tCKDD
Clock Falling to DATA
High Impedance
70
100
ns
ns
tCKSP
Clock Falling to Sample Mode
5
tCKPD
Clock Falling to Power-Down Mode
50
ns
tCVHD
CONV Falling to Hold Mode
(Aperture Delay)
5
ns
ns
tCVSP
CONV Rising to Sample Mode
5
tCVPU
CONV Rising to Full Power-up
50
tCVDD
CONV Changing State to DATA
High Impedance
70
tCVPD
CONV Changing State to
Power-Down Mode
50
tDRP
CONV Falling to Start of CLK
(for hold droop < 0.1 LSB)
ns
ns
100
ns
ns
5
µs
Note: (1) This timing is not required under some situations. See text for more information.
TABLE I. Timing Specifications (TA = –40°C to +85°C,
CLOAD = 30pF).
tCKP
tCKH
tCKL
CLK
tCKDS
DATA
FIGURE 2. Serial Data and Clock Timing.
®
ADS7835
8
tCKDH
clock cycle is the first of the conversion.)
tween conversions.
In addition, if CONV is completely asynchronous to CLK
and CLK is continuous, there is the possibility that CLK will
transition just prior to CONV going LOW. If this occurs
faster than the 10ns indicated by tCKCH, there is a chance that
some digital feedthrough may be coupled onto the hold
capacitor. This could cause a small offset error for that
particular conversion.
Figure 4 shows the typical method for placing the A/D into
the power-down mode. If CONV is kept LOW during the
conversion and is LOW at the start of the 13th clock cycle,
the device enters the power-down mode. It remains in this
mode until the rising edge of CONV. Note that CONV must
be HIGH for at least tACQ in order to sample the signal
properly as well as to power-up the internal nodes.
Thus, there are two basic ways to operate the ADS7835.
CONV can be synchronous to CLK and CLK can be continuous. This would be the typical situation when interfacing
the converter to a digital signal processor. The second
method involves having CONV asynchronous to CLK and
gating the operation of CLK (a non-continuous clock). This
method would be more typical of an SPI-like interface on a
microcontroller. This method would also allow CONV to be
generated by a trigger circuit and to initiate (after some
delay) the start of CLK. These two methods are covered
under the DSP Interfacing and SPI Interfacing sections of
this data sheet.
There are two different methods for clocking the ADS7835.
The first involves scaling the CLK input in relation to the
conversion rate. For example, an 8MHz input clock and the
timing shown in Figure 3 results in a 500kHz conversion
rate. Likewise, a 1.6MHz clock would result in a 100kHz
conversion rate. The second method involves keeping the
clock input as close to the maximum clock rate as possible
and starting conversions as needed. This timing is similar to
that shown in Figure 4. As an example, a 50kHz conversion
rate would require 160 clock periods per conversion instead
of the 16 clock periods used at 500kHz.
The main distinction between the two is the amount of time
that the ADS7835 remains in power-down. In the first mode,
the converter only remains in power-down for a small
number of clock periods (depending on how many clock
periods there are per each conversion). As the conversion
rate scales, the converter always spends the same percentage
of time in power-down. Since less power is drawn by the
digital logic, there is a small decrease in power consumption, but it is very slight. This effect can be seen in the
POWER-DOWN TIMING
The conversion timing shown in Figure 3 does not result in
the ADS7835 going into the power-down mode. If the
conversion rate of the device is high (approaching 500kHz),
there is very little power that can be saved by using the
power-down mode. However, since the power-down mode
incurs no conversion penalty (the very first conversion is
valid) at lower sample rates, significant power can be saved
by allowing the device to go into power-down mode be-
tCVL
tCVCK
CONV
tCKCS
tCKCH
CLK
14
15
16
1
2
3
4
11
12
13
14
15
16
1
(1)
tCKDE
D11
(MSB)
DATA
tCKDD
D10
D9
D2
D1
D0
(LSB)
tACQ
tCVHD
SAMPLE/HOLD
MODE
tCKSP
HOLD
SAMPLE
SAMPLE
HOLD
(2)
tCONV
INTERNAL
CONVERSION
STATE
IDLE
CONVERSION IN PROGRESS
IDLE(3)
NOTES: (1) Clock periods 14 and 15 are shown for clarity, but are not required for proper operation of the ADS7835, provided that the
minimum tACQ time is met. The CLK input may remain HIGH or LOW during this period. (2) The transition from sample mode to hold
mode occurs on the falling edge of CONV. This transition is not dependent on CLK. (3) The device remains fully powered when
operated as shown. If the sample time is longer than 3 clock periods, power consumption can be reduced by allowing the device to
enter a power-down mode. See the Power-Down Timing section for more information.
FIGURE 3. Basic Conversion Timing.
®
9
ADS7835
CONV
1
CLK
2
D11
(MSB)
DATA
3
12
D10
D1
13
D0
(LSB)
tCVSP
SAMPLE/HOLD
SAMPLE
MODE
tACQ
HOLD
SAMPLE
HOLD
(3)
INTERNAL
CONVERSION
STATE
IDLE
CONVERSION IN PROGRESS
IDLE
tCKPD
tCVPU
POWER MODE
FULL POWER
LOW POWER
(1)
FULL POWER
(2)
NOTES: (1) The low power mode (“power-down”) is entered when CONV remains LOW during the conversion and is still LOW at the
start of the 13th clock cycle. (2) The low power mode is exited when CONV goes HIGH. (3) When in power-down, the transition from
hold mode to sample mode is initiated by CONV going HIGH.
FIGURE 4. Power-Down Timing.
tCVH
CONV
tCKCH
1
CLK
2
3
12
13
14
23
24
D10
D11
(MSB)
tCKCS
D11
(MSB)
DATA
D10
D0
(LSB)
D1
D1
tCVDD
(1)
SAMPLE/HOLD
SAMPLE
MODE
INTERNAL
CONVERSION
STATE
IDLE
LOW...
(2)
HOLD
CONVERSION IN PROGRESS
IDLE
tCVPD
POWER MODE
FULL POWER
LOW POWER
(3)
NOTES: (1) The serial data can be transmitted LSB-first by pulling CONV LOW during the 13th clock cycle. (2) After the MSB has been
transmitted, the DATA output pin will remain LOW until CONV goes HIGH. (3) When CONV is taken LOW to initiate the LSB first transfer,
the converter enters the power-down mode.
FIGURE 5. Serial Data “LSB-First” Timing.
typical performance curve “Supply Current vs Sample Rate.”
in power-down an increasing percentage of time. This reduces total power consumption by a considerable amount.
For example, a 50kHz conversion rate results in roughly
1/10 of the power (minus the reference) that is used at a
In contrast, the second method (clocking at a fixed rate)
means that each conversion takes X clock cycles. As the
time between conversions get longer, the converter remains
®
ADS7835
10
500kHz conversion rate.
HIGH.
SHORT-CYCLE TIMING
fSAMPLE
POWER WITH
CLK = 16 • fSAMPLE
POWER WITH
CLK = 8MHz
500kHz
17.5mW
17.5mW
250kHz
16.5mW
13.5mW
100kHz
15.5mW
10.5mW
The conversion currently in progress can be “short-cycled”
with the technique shown in Figure 6. This term means that
the conversion will terminate immediately, before all 12 bits
have been decided. This can be a very useful feature when
a resolution of 12 bits is not needed. An example would be
when the converter is being used to monitor an input voltage
until some condition is met. At that time, the full resolution
of the converter would then be used. Short-cycling the
conversion can result in a faster conversion rate or lower
power dissipation.
TABLE II. Power Consumption versus CLK Input.
Table II offers a look at the two different modes of operation
and the difference in power consumption.
LSB-FIRST DATA TIMING
There are several very important items shown in Figure 6.
The conversion currently in progress is terminated when
CONV is taken HIGH during the conversion and then taken
LOW prior to tCKCH before the start of the 13th clock cycle.
Note that if CONV goes LOW during the 13th clock cycle,
the LSB-first mode will be entered (Figure 5). Additionally,
when CONV goes LOW, the DATA output immediately
transitions to high impedance. If the output bit that is present
during that clock period is needed, CONV must not go LOW
until the bit has been properly latched into the receiving
Figure 5 shows a method to transmit the digital result in a
LSB format. This mode is entered when CONV is pulled
HIGH during the conversion (before the end of the 12th
clock) and then pulled LOW during the 13th clock (when
D0, the LSB, is being transmitted). The next 11 clocks then
repeat the serial data, but in an LSB-first format. The
converter enters the power-down mode during the 13th
clock and resumes normal operation when CONV goes
tCVL
(1)
CONV
tCVH
1
CLK
2
3
4
5
6
7
tCVDD
D11
(MSB)
DATA
SAMPLE/HOLD
MODE
INTERNAL
CONVERSION
STATE
D10
D9
D8
SAMPLE
IDLE
D7
D6
HOLD
CONVERSION IN PROGRESS
IDLE
tCVPD
POWER MODE
FULL POWER
LOW POWER
NOTE: (1) The conversion currently in progress can be stopped by pulling CONV LOW during the conversion. This must occur at
least tCKCS prior to the start of the 13th clock cycle. The DATA output pin will tri-state and the device will enter the power-down
mode when CONV is pulled LOW.
FIGURE 6. Short-Cycle Timing.
®
11
ADS7835
logic.
Figure 7 shows a timing diagram that might be used with a
typical digital signal processor such as a TI DSP. For the
Buffered Serial Port (BSP) on the TMS320C54X family,
CONV would tied to BFSX, CLK would be tied to BCLKX,
and DATA would be tied to BDR.
DATA FORMAT
The ADS7835 output data is in Binary Two’s Complement
format as shown in Table III. This table shows the ideal
output code for the given input voltage and does not include
DESCRIPTION
ANALOG INPUT
Full-Scale Input
Range
Least Significant Bit
(LSB)(2)
–VREF to +VREF(1)
(–VREF to +VREF)/4096
+Full Scale
Mid-Scale
Mid-Scale –1LSB
–Full Scale
SPI/QSPI INTERFACING
DIGITAL OUTPUT
2.49878V
0V
–0.00122V
–2.49878V
Figure 8 shows the timing diagram for a typical Serial
Peripheral Interface (SPI) or Queued Serial Peripheral Interface (QSPI). Such interfaces are found on a number of
microcontrollers from various manufacturers. CONV would
be tied to a general purpose I/O pin (SPI) or to a PCX pin
(QSPI), CLK would be tied to the serial clock, and DATA
would be tied to the serial input data pin such as MISO
(Master In Slave Out).
BINARY TWO’S
COMPLEMENT
BINARY
HEX
CODE
CODE
0111
0000
1111
1000
1111
0000
1111
0000
1111
0000
1111
0000
7FF
000
FFF
800
NOTES: (1) –2.5V to +2.5V when the internal reference is used. (2) 1.22mV
with a 2.5V reference.
Note the time tDRP shown in Figure 8. This represents the
maximum amount of time between CONV going LOW and
the start of the conversion clock. Since CONV going LOW
places the S/H in the hold mode and because the hold
capacitor loses charge over time, there is a requirement that
time tDRP be met as well as the maximum clock period
TABLE III. Ideal Input Voltages and Output Codes.
the effects of offset, gain, or noise.
DSP INTERFACING
CONV
CLK
15
16
1
2
D11
(MSB)
DATA
3
D10
12
D1
13
14
15
D0
(LSB)
16
1
2
D11
(MSB)
3
4
D10
D9
FIGURE 7. Typical DSP Interface Timing.
tDRP
tACQ
CONV
1
CLK
2
3
D11
(MSB)
DATA
4
D10
13
D1
14
D0
(LSB)
FIGURE 8. Typical SPI/QSPI Interface Timing.
®
ADS7835
12
15
16
1
2
3
D11
(MSB)
(tCKP).
larger capacitor and a 5Ω or 10Ω series resistor may be used
to lowpass filter a noisy supply.
LAYOUT
The ADS7835 draws very little current from an external
reference on average as the reference voltage is internally
buffered. However, glitches from the conversion process
appear at the VREF input and the reference source must be
able to handle this. Whether the reference is internal or
external, the VREF pin should be bypassed with a 0.1µF
capacitor. An additional larger capacitor may also be used,
if desired. If the reference voltage is external and originates
from an op amp, make sure that it can drive the bypass
capacitor or capacitors without oscillation.
For optimum performance, care should be taken with the
physical layout of the ADS7835 circuitry. This is particularly true if the CLK input is approaching the maximum
input rate.
The basic SAR architecture is sensitive to glitches or sudden
changes on the power supply, reference, ground connections, and digital inputs that occur just prior to latching the
output of the analog comparator. Thus, during any single
conversion for an n-bit SAR converter, there are n “windows” in which large external transient voltages can easily
affect the conversion result. Such glitches might originate
from switching power supplies, nearby digital logic, and
high power devices. The degree of error in the digital output
depends on the reference voltage, layout, and the exact
timing of the external event. The error can change if the
external event changes in time with respect to the CLK
input.
The GND pin should be connected to a clean ground point.
In many cases, this will be the “analog” ground. Avoid
connections which are too near the grounding point of a
microcontroller or digital signal processor. If needed, run a
ground trace directly from the converter to the power supply
entry point. The ideal layout will include an analog ground
plane dedicated to the converter and associated analog
circuitry.
With this in mind, power to the ADS7835 should be clean
and well bypassed. A 0.1µF ceramic bypass capacitor should
be placed as close to the device as possible. In addition, a
1µF to 10µF capacitor is recommended. If needed, an even
®
13
ADS7835