BB ADS7820

®
ADS
ADS7820
782
0
ADS
782
0
12-Bit 10µs Sampling CMOS
ANALOG-to-DIGITAL CONVERTER
FEATURES
DESCRIPTION
● 100kHz min SAMPLING RATE
● 0 to +5V INPUT RANGE
The ADS7820 is a complete 12-bit sampling A/D
using state-of-the-art CMOS structures. It contains a
complete 12-bit, capacitor-based SAR A/D with S/H,
reference, clock, interface for microprocessor use, and
three-state output drivers.
● 72dB min SINAD WITH 45kHz INPUT
● ±1/2 LSB max INL AND DNL
● SINGLE +5V SUPPLY OPERATION
● PIN-COMPATIBLE WITH 16-BIT ADS7821
The ADS7820 is specified at a 100kHz sampling rate,
and guaranteed over the full temperature range. Lasertrimmed scaling resistors provide a 0 to +5V input
range, with power dissipation under 100mW.
The 28-pin ADS7820 is available in a plastic 0.3" DIP
and in an SOIC, both fully specified for operation over
the industrial –40°C to +85°C range.
● USES INTERNAL OR EXTERNAL
REFERENCE
● COMPLETE WITH S/H, REF, CLOCK, ETC.
● FULL PARALLEL DATA OUTPUT
● 100mW max POWER DISSIPATION
● 28-PIN 0.3" PLASTIC DIP AND SOIC
Clock
Successive Approximation Register and Control Logic
R/C
CS
BYTE
BUSY
CDAC
5kΩ
0 to +5V Input
6.66kΩ
20kΩ
Comparator
Output
Latches
and
Three
State
Drivers
Three
State
Parallel
Data
Bus
CAP
Buffer
Internal
+2.5V Ref
4kΩ
REF
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 • Cable: BBRCORP • Telex: 066-6491 • FAX: (520) 889-1510 • Immediate Product Info: (800) 548-6132
®
©
1996 Burr-Brown Corporation
1
PDS-1322
Printed in U.S.A. June, 1996
ADS7820
SPECIFICATIONS
ELECTRICAL
TA = –40°C to +85°C, fS = 100kHz, VDIG = VANA = +5V, using internal reference, unless otherwise specified.
ADS7820P/U
PARAMETER
CONDITIONS
MIN
TYP
RESOLUTION
ADS7820PB/UB
MAX
MIN
TYP
12
ANALOG INPUT
Voltage Ranges
Impedance
Capacitance
DC ACCURACY
Integral Linearity Error
Differential Linearity Error
No Missing Codes
Transition Noise(2)
Full Scale Error(3,4)
Full Scale Error Drift
Full Scale Error(3,4)
Full Scale Error Drift
Offset Error
Offset Error Drift
Power Supply Sensitivity
(VDIG = VANA = VD)
AC ACCURACY
Spurious-Free Dynamic Range
Total Harmonic Distortion
Signal-to-(Noise+Distortion)
Signal-to-Noise
Full-Power Bandwidth(6)
SAMPLING DYNAMICS
Aperture Delay
Transient Response
Overvoltage Recovery(7)
REFERENCE
Internal Reference Voltage
Internal Reference Source Current
(Must use external buffer.)
External Reference Voltage Range
for Specified Linearity
External Reference Current Drain
5.7
Acquire and Convert
±7
±2
±2
+4.75V < VD < +5.25V
45kHz
45kHz
45kHz
45kHz
±0.5
±5
±0.5
✱
✱
µs
µs
kHz
±0.5
±0.5
LSB(1)
LSB
Bits
LSB
%
ppm/°C
%
ppm/°C
mV
ppm/°C
LSB
±0.25
±0.25
✱
±8
±4
✱
±0.75
±0.5
✱
80
✱
–80
70
70
72
72
✱
250
✱
40
FS Step
✱
2
✱
150
dB(5)
dB
dB
dB
kHz
ns
µs
ns
2.48
2.5
1
2.52
✱
✱
✱
✱
V
µA
2.3
2.5
2.7
✱
✱
✱
V
✱
µA
✱
✱
✱
✱
V
V
µA
µA
✱
✱
V
V
µA
15
✱
pF
83
83
✱
✱
ns
ns
Ext. 2.5000V Ref
100
–0.3
+2.0
Output Capacitance
V
kΩ
pF
✱
✱
Guaranteed
0.1
fIN =
fIN =
fIN =
fIN =
Bits
✱
±1.0
±1.0
DIGITAL INPUTS
Logic Levels
VIL
VIH
IIL
IIH
DIGITAL OUTPUTS
Data Format
Data Coding
VOL
VOH
Leakage Current
✱
8
10
100
Ext. 2.5000V Ref
Ext. 2.5000V Ref
UNITS
✱
✱
✱
✱
0 to +5
10
35
THROUGHPUT SPEED
Conversion Time
Complete Cycle
Throughput Rate
MAX
ISINK = 1.6mA
ISOURCE = 500µA
High-Z State,
VOUT = 0V to VDIG
High-Z State
+4
DIGITAL TIMING
Bus Access Time
Bus Relinquish Time
+0.8
VD +0.3V
±10
±10
✱
✱
Parallel 12 bits
Straight Binary
+0.4
✱
±5
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.
®
ADS7820
2
SPECIFICATIONS (CONT)
ELECTRICAL
TA = –40°C to +85°C, fS = 100kHz, VDIG = VANA = +5V, using internal reference, unless otherwise specified.
ADS7820P/U
PARAMETER
POWER SUPPLIES
Specified Performance
VDIG
VANA
+IDIG
+IANA
ADS7820PB/UB
CONDITIONS
MIN
TYP
MAX
Must be ≤ VANA
+4.75
+4.75
+5
+5
0.3
16
+5.25
+5.25
Power Dissipation
f S = 100kHz
MIN
✱
✱
TYP
✱
✱
✱
✱
100
TEMPERATURE RANGE
Specified Performance
Derated Performance
Storage
Thermal Resistance (θJA)
Plastic DIP
SOIC
–40
–55
–65
+85
+125
+150
75
75
✱
✱
✱
✱
✱
MAX
UNITS
✱
✱
V
V
mA
mA
✱
mW
✱
✱
✱
°C
°C
°C
°C/W
°C/W
NOTES: (1) LSB means Least Significant Bit. For the 12-bit, 0 to +5V input ADS7820, one LSB is 1.22mV. (2) Typical rms noise at worst case transitions and
temperatures. (3) Adjustable to zero with external potentiometer as shown in Figure 4b. (4) Full scale error is the worst case of Full Scale untrimmed deviation from
ideal last code transition divided by the transition voltage and includes the effect of offset error. (5) All specifications in dB are referred to a full-scale input. (6) FullPower Bandwidth defined as Full-Scale input frequency at which Signal-to-(Noise + Distortion) degrades to 60dB, or 10 bits of accuracy. (7) Recovers to specified
performance after 2 x FS input overvoltage.
ABSOLUTE MAXIMUM RATINGS
ELECTROSTATIC
DISCHARGE SENSITIVITY
Analog Inputs: VIN .................................................... –0.7V to +VANA +0.3V
REF .................................... +VANA +0.3V to AGND2 –0.3V
CAP ........................................... Indefinite Short to AGND2
Momentary Short to VANA
Ground Voltage Differences: DGND, AGND1, AGND2 ................... ±0.3V
VANA ....................................................................................................... 7V
VDIG to VANA ..................................................................................... +0.3V
VDIG ....................................................................................................... 7V
Digital Inputs ............................................................ –0.3V to +VDIG +0.3V
Maximum Junction Temperature ................................................... +165°C
Internal Power Dissipation ............................................................. 825mW
Lead Temperature (soldering, 10s) ................................................ +300°C
Electrostatic discharge can cause damage ranging from performance degradation to complete device failure. BurrBrown Corporation recommends that all integrated circuits
be handled and stored using appropriate ESD protection
methods.
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.
PACKAGE INFORMATION
PRODUCT
PACKAGE
PACKAGE DRAWING
NUMBER(1)
ADS7820P
ADS7820PB
ADS7820U
ADS7820UB
Plastic DIP
Plastic DIP
SOIC
SOIC
246
246
217
217
NOTE: (1) For detailed drawing and dimension table, please see end of data
sheet, or Appendix D of Burr-Brown IC Data Book.
ORDERING INFORMATION
PRODUCT
ADS7820P
ADS7820PB
ADS7820U
ADS7820UB
MAXIMUM
LINEARITY
ERROR (LSB)
MINIMUM
SIGNAL-TO(NOISE +
DISTORTION)
RATIO (dB)
±1.0
±0.5
±1.0
±0.5
70
72
70
72
SPECIFICATION
TEMPERATURE
RANGE
PACKAGE
–40°C
–40°C
–40°C
–40°C
Plastic DIP
Plastic DIP
SOIC
SOIC
to
to
to
to
+85°C
+85°C
+85°C
+85°C
®
3
ADS7820
PIN CONFIGURATION
VIN
1
28 VDIG
AGND1
2
27 VANA
REF
3
26 BUSY
CAP
4
25 CS
AGND2
5
24 R/C
D11 (MSB)
6
23 BYTE
D10
7
22 DZ
ADS7820
D9
8
21 DZ
D8
9
20 DZ
D7 10
19 DZ
D6 11
18 D0 (LSB)
D5 12
17 D1
D4 13
16 D2
DGND 14
15 D3
PIN ASSIGNMENTS
PIN #
NAME
DESCRIPTION
1
VIN
2
AGND1
Analog Input. Full-scale input range is 0 to +5V.
3
REF
Reference Input/Output. Outputs internal reference of +2.5V nominal. Can also be driven by external system reference. In both
cases, connect to ground with a 2.2µF Tantalum capacitor.
4
CAP
Reference Buffer Capacitor. 2.2µF Tantalum to ground.
5
AGND2
6
D11 (MSB)
7
D10
Data Bit 10. Hi-Z state when CS is HIGH, or when R/C is LOW.
8
D9
Data Bit 9. Hi-Z state when CS is HIGH, or when R/C is LOW.
9
D8
Data Bit 8. Hi-Z state when CS is HIGH, or when R/C is LOW.
10
D7
Data Bit 7. Hi-Z state when CS is HIGH, or when R/C is LOW.
11
D6
Data Bit 6. Hi-Z state when CS is HIGH, or when R/C is LOW.
12
D5
Data Bit 5. Hi-Z state when CS is HIGH, or when R/C is LOW.
13
D4
Data Bit 4. Hi-Z state when CS is HIGH, or when R/C is LOW.
14
DGND
15
D3
Data Bit 3. Hi-Z state when CS is HIGH, or when R/C is LOW.
16
D2
Data Bit 2. Hi-Z state when CS is HIGH, or when R/C is LOW.
17
D1
Data Bit 1. Hi-Z state when CS is HIGH, or when R/C is LOW.
18
D0 (LSB)
19
DZ
Data Zero. LOW when CS LOW and R/C HIGH. Hi-Z state when CS is HIGH, or when R/C is LOW.
20
DZ
Data Zero. LOW when CS LOW and R/C HIGH. Hi-Z state when CS is HIGH, or when R/C is LOW.
21
DZ
Data Zero. LOW when CS LOW and R/C HIGH. Hi-Z state when CS is HIGH, or when R/C is LOW.
22
DZ
Data Zero. LOW when CS LOW and R/C HIGH. Hi-Z state when CS is HIGH, or when R/C is LOW.
23
BYTE
Byte Select. With BYTE LOW, data will be output as indicated above, causing pin 6 (D11) to output the MSB, and pin 18 (D0) to
output the LSB. Pins 19 to 22 will output LOWs. With BYTE HIGH, the top and bottom 8 bits of data will be switched, so that pin 6
outputs data bit 3, pin 9 outputs data bit 0 (LSB), pin 10 to 13 output LOWs, pin 15 outputs data bit 11 (MSB) and pin 22 outputs
data bit 4.
24
R/C
Read/Convert input. With CS LOW, a falling edge on R/C puts the internal sample/hold into the hold state and starts a conversion.
With CS LOW, a rising edge on R/C enables the output data bits.
25
CS
Chip Select. Internally OR’d with R/C. With R/C LOW, a falling edge on CS will initiate a conversion. With R/C HIGH, a falling edge
on CS will enable the output data bits.
26
BUSY
27
VANA
Analog Supply Input. Nominally +5V. Connect directly to pin 28. Decouple to ground with 0.1µF ceramic and 10µF Tantalum
capacitors.
28
VDIG
Digital Supply Input. Nominally +5V. Connect directly to pin 27. Must be ≤ VANA.
Analog Ground. Used internally as ground reference point.
Analog Ground.
Data Bit 11. Most Significant Bit (MSB) of conversion results. Hi-Z state when CS is HIGH, or when R/C is LOW.
Digital Ground.
Data Bit 0. Lease Significant Bit (LSB) of conversion results. Hi-Z state when CS is HIGH, or when R/C is LOW.
Busy Output. Falls when a conversion is started, and remains LOW until the conversion is completed and the data is latched into the
output register. With CS LOW and R/C HIGH, output data will be valid when BUSY rises, so that the rising edge can be used to
latch the data. CS or R/C must be high when BUSY rises, or another conversion will start, without time for signal acquisition.
®
ADS7820
4
TYPICAL PERFORMANCE CURVES
At TA = –40°C to +85°C, fS = 100kHz, VDIG = VANA = +5V, using internal reference, unless otherwise specified.
CONVERSION TIME vs TEMPERATURE
6.2
2.515
6.1
Conversion Time (µs)
Internal Reference (V)
INTERNAL REFERENCE VOLTAGE vs TEMPERATURE
2.52
2.51
2.505
2.5
2.495
2.49
2.485
6
5.9
5.8
5.7
5.6
5.5
2.48
5.4
–40
–15
10
35
60
85
–40
ADS7820 AT +25°C
12-Bit LSBs
60
85
Min/Max DNL Errors
–0.073 at 00512
0.087 at 02047
12-Bit LSBs
1024
35
ADS7820 AT +25°C
0.2
+0.1
0
–0.1
–0.2
512
10
Temperature (°C)
Min/Max INL Errors
–0.051 at 03962
0.086 at 02048
0
–15
Temperature (°C)
1536
2048
2560
3072
3584
4096
0.2
+0.1
0
–0.1
–0.2
0
Decimal Code
512
1024
1536
2048
2560
3072
3584
4096
Decimal Code
®
5
ADS7820
BASIC OPERATION
CS and R/C are internally OR’d and level triggered. There
is not a requirement which input goes LOW first when
initiating a conversion. If, however, it is critical that CS or
R/C initiates conversion ‘n’, be sure the less critical input is
LOW at least 10ns prior to the initiating input.
Figure 1 shows a basic circuit to operate the ADS7820 with
a full parallel data output. Taking R/C (pin 24) LOW for a
minimum of 40ns (5.4µs max) will initiate a conversion.
BUSY (pin 26) will go LOW and stay LOW until the
conversion is completed and the output registers are updated. Data will be output in Straight Binary with the MSB
on pin 6. BUSY going HIGH can be used to latch the data.
All convert commands will be ignored while BUSY is
LOW.
To reduce the number of control pins, CS can be tied LOW
using R/C to control the read and convert modes. However,
the output will become active whenever R/C goes HIGH.
Refer to the Reading Data section.
The ADS7820 will begin tracking the input signal at the end
of the conversion. Allowing 10µs between convert commands assures accurate acquisition of a new signal.
CS
R/C
BUSY
OPERATION
1
X
X
None. Databus is in Hi-Z state.
↓
0
1
Initiates conversion “n”. Databus remains
in Hi-Z state.
STARTING A CONVERSION
0
↓
1
Initiates conversion “n”. Databus enters Hi-Z
state.
The combination of CS (pin 25) and R/C (pin 24) LOW for
a minimum of 40ns immediately puts the sample/hold of the
ADS7820 in the hold state and starts conversion ‘n’. BUSY
(pin 26) will go LOW and stay LOW until conversion ‘n’ is
completed and the internal output register has been updated.
All new convert commands during BUSY LOW will be
ignored. CS and/or R/C must go HIGH before BUSY goes
HIGH or a new conversion will be initiated without sufficient time to acquire a new signal.
0
1
↑
Conversion “n” completed. Valid data from
conversion “n” on the databus.
↓
1
1
Enables databus with valid data from
conversion “n”.
↓
1
0
Enables databus with valid data from
conversion “n-1”(1). Conversion n in process.
0
↑
0
Enables databus with valid data from
conversion “n-1”(1). Conversion “n” in process.
0
0
↑
New conversion initiated without acquisition
of a new signal. Data will be invalid. CS and/or
R/C must be HIGH when BUSY goes HIGH.
X
X
0
New convert commands ignored. Conversion
“n” in process.
The ADS7820 will begin tracking the input signal at the end
of the conversion. Allowing 10µs between convert commands assures accurate acquisition of a new signal. Refer to
Table I for a summary of CS, R/C, and BUSY states and
Figures 3 through 5 for timing diagrams.
0 to +5V
+
NOTE: (1) See Figures 2 and 3 for constraints on data valid from
conversion “n-1”.
Table I. Control Line Functions for “Read” and “Convert”.
1
28
2
27
2.2µF
3
26
4
25
5
24
B11 (MSB)
6
23
B10
7
2.2µF
+
+
0.1µF
+
+5V
10µF
Convert Pulse
22
LOW
ADS7820
B9
8
21
LOW
B8
9
20
LOW
B7
10
19
LOW
B6
11
18
B0 (LSB)
B5
12
17
B1
B4
13
16
B2
14
15
B3
FIGURE 1. Basic Operation (Byte Low).
®
ADS7820
6
40ns min
5.4µs max
READING DATA
PARALLEL OUTPUT (During a Conversion)
After conversion ‘n’ has been initiated, valid data from
conversion ‘n-1’ can be read and will be valid up to 16µs
after the start of conversion ‘n’. Do not attempt to read data
from 16µs after the start of conversion ‘n’ until BUSY (pin
26) goes HIGH; this may result in reading invalid data.
Refer to Table III and Figures 3 and 5 for timing specifications.
The ADS7820 outputs full or byte-reading parallel data in
Straight Binary data output format. The parallel output will
be active when R/C (pin 24) is HIGH and CS (pin 25) is
LOW. Any other combination of CS and R/C will tri-state
the parallel output. Valid conversion data can be read in a
full parallel, 12-bit word or two 8-bit bytes on pins 6-13 and
pins 15-22. BYTE (pin 23) can be toggled to read both bytes
within one conversion cycle. Refer to Table II for ideal
output codes and Figure 2 for bit locations relative to the
state of BYTE.
Note! For the best possible performance, data should not be
read during a conversion. The switching noise of the asynchronous data transfer can cause digital feedthrough degrading the converter’s performance.
The number of control lines can be reduced by tieing CS
LOW while using R/C to initiate conversions and activate
the output mode of the converter. See Figure 3.
DIGITAL OUTPUT
STRAIGHT BINARY
DESCRIPTION
ANALOG INPUT
Full Scale Range
0 to +5V
Least Significant
Bit (LSB)
1.22mV
Full Scale
Midscale
One LSB below
Midscale
Zero Scale
4.99878V
BINARY CODE
HEX CODE
SYMBOL
DESCRIPTION
1111 1111 1111
FFF
t1
Convert Pulse Width
5400
ns
Data Valid Delay after R/C LOW
8
µs
65
8
ns
µs
MIN TYP MAX UNITS
40
2.5V
1000 0000 0000
800
t2
2.49878V
0111 1111 1111
7FF
t3
t4
BUSY Delay from R/C LOW
BUSY LOW
0V
0000 0000 0000
0
t5
BUSY Delay after
End of Conversion
220
ns
t6
Aperture Delay
40
ns
t7
Conversion Time
7.6
t8
Acquisition Time
Table II. Ideal Input Voltages and Output Codes.
PARALLEL OUTPUT (After a Conversion)
After conversion ‘n’ is completed and the output registers
have been updated, BUSY (pin 26) will go HIGH. Valid data
from conversion ‘n’ will be available on D11-D0 (pin 6-13
and 15-18 when BYTE is LOW). BUSY going HIGH can be
used to latch the data. Refer to Table III and Figures 3 and
5 for timing specifications.
8
µs
2
µs
t9
Bus Relinquish Time
10
35
t10
BUSY Delay after Data Valid
50
200
ns
t11
Previous Data Valid
after R/C LOW
7.4
µs
83
ns
µs
t7 + t6
Throughput Time
t12
R/C to CS Setup Time
10
ns
t13
Time Between Conversions
10
µs
t14
Bus Access Time
and BYTE Delay
10
9
10
83
ns
TABLE III. Conversion Timing.
BYTE LOW
BYTE HIGH
+5V
Bit 11 (MSB)
6
Bit 10
7
23
Bit 3
6
22 LOW
Bit 2
7
ADS7820
23
22 Bit 4
ADS7820
Bit 9
8
21 LOW
Bit 1
8
21 Bit 5
Bit 8
9
20 LOW
Bit 0 (LSB)
9
20 Bit 6
Bit 7 10
19 LOW
LOW 10
19 Bit 7
Bit 6 11
18 Bit 0 (LSB)
LOW 11
18 Bit 8
Bit 5 12
17 Bit 1
LOW 12
17 Bit 9
Bit 4 13
16 Bit 2
LOW 13
16 Bit 10
14
15 Bit 3
14
15 Bit 11
FIGURE 2. Bit Locations Relative to State of BYTE (pin 23).
®
7
ADS7820
t1
R/C
t13
t2
t4
BUSY
t3
t6
t5
Convert
Acquire
MODE
Acquire
t7
Previous
Data Valid
DATA BUS
t8
Previous
Data Valid
Hi-Z
t9
Convert
Data Valid
Not Valid
Hi-Z
t10
t11
FIGURE 3. Conversion Timing with Outputs Enabled after Conversion (CS Tied LOW.)
t12
t12
t12
t12
R/C
t1
CS
t3
t4
BUSY
t6
MODE
Convert
Acquire
Acquire
t7
Hi-Z State
DATA BUS
Data Valid
t9
t14
FIGURE 4. Using CS to Control Conversion and Read Timing.
t12
t12
R/C
CS
BYTE
Pins 6 - 13
Hi-Z
High Byte
t14
Pins 15 - 22
Hi-Z
t14
Low Byte
FIGURE 5. Using CS and BYTE to Control Data Bus.
®
ADS7820
Low Byte
8
High Byte
Hi-Z
t9
Hi-Z
Hi-Z State
Data Valid
INPUT RANGE
REF
The ADS7820 offers a standard 0V to 5V input range.
Figure 6 shows the required circuit connections for the
ADS7820 with and without the gain adjustment hardware.
Adjustments for offset and gain are described in the calibration section of this data sheet.
REF (pin 3) is an input for an external reference or the output
for the internal 2.5V reference. A 2.2µF capacitor should be
connected as close to the REF pin as possible. The capacitor
and the output resistance of REF create a low pass filter to
bandlimit noise on the reference. Using a smaller value
capacitor will introduce more noise to the reference degrading the SNR and SINAD. The REF pin should not be used
to drive external AC or DC loads.
CALIBRATION
The range for the external reference is 2.3V to 2.7V and
determines the actual LSB size. Increasing the reference
voltage will increase the full scale range and the LSB size of
the converter which can improve the SNR.
The ADS7820 can be trimmed in hardware or software. There
is no external, offset adjustment. If offset adjustment is required, an op amp featuring an offset trim pin should be used
to drive the ADS7820. The offset should be trimmed before
the gain since the offset directly affects the gain. To achieve
optimum performance, several iterations may be required.
CAP
CAP (pin 4) is the output of the internal reference buffer. A
2.2µF capacitor should be placed as close to the CAP pin as
possible to provide optimum switching currents for the
CDAC throughout the conversion cycle and compensation
for the output of the internal buffer. Using a capacitor any
smaller than 1µF can cause the output buffer to oscillate and
may not have sufficient charge for the CDAC. Capacitor
values larger than 2.2µF will have little affect on improving
performance.
GAIN ADJUSTMENT
To calibrate the gain of the ADS7820, a 576kΩ resistor must
be tied between the REF pin and a 5V potentiometer (see
Figure 6b). The calibration range is ±15mV for the gain.
REFERENCE
The ADS7820 can operate with its internal 2.5V reference or
an external reference. By applying an external reference to
pin 3, the internal reference can be bypassed. The reference
voltage at REF is buffered internally with the output on CAP
(pin 4).
The output of the buffer is capable of driving up to 2mA of
current to a DC load. DC loads requiring more than 2mA of
current from the CAP pin will begin to degrade the linearity
of the ADS7820. Using an external buffer will allow the
internal reference to be used for larger DC loads and AC
loads. Do not attempt to directly drive an AC load with the
output voltage on CAP. This will cause performance degradation of the converter.
The internal reference has an 8 ppm/°C drift (typical) and
accounts for approximately 20% of the full scale error
(FSE = ±0.5% for low grade, ±0.25% for high grade).
a) WITHOUT EXTERNAL GAIN ADJUSTMENT
1
0 to +5V
2
2.2µF
+
b) WITH EXTERNAL GAIN ADJUSTMENT
2
AGND1
2.2µF
+5V
3
1
0 to +5V
VIN
+
3
REF
VIN
AGND1
REF
576kΩ
4
2.2µF
4
50kΩ
CAP
Gain
+
2.2µF
5
5
AGND2
CAP
+
AGND2
FIGURE 6. Circuit Diagram With and Without External Gain Adjustment.
®
9
ADS7820
LAYOUT
SIGNAL CONDITIONING
The FET switches used for the sample hold on many CMOS
A/D converters release a significant amount of charge injection which can cause the driving op amp to oscillate. The
FET switch on the ADS7820, compared to the FET switches
on other CMOS A/D converters, releases 5%-10% of the
charge. There is also a resistive front end which attenuates
any charge which is released. The end result is a minimal
requirement for the anti-alias filter on the front end. Any op
amp sufficient for the signal in an application will be
sufficient to drive the ADS7820.
POWER
For optimum performance, tie the analog and digital power
pins to the same +5V power supply and tie the analog and
digital grounds together. As noted in the electrical specifications, the ADS7820 uses 90% of its power for the analog
circuitry. The ADS7820 should be considered as an analog
component.
The +5V power for the A/D should be separate from the +5V
used for the system’s digital logic. Connecting VDIG (pin 28)
directly to a digital supply can reduce converter performance
due to switching noise from the digital logic. For best
performance, the +5V supply can be produced from whatever analog supply is used for the rest of the analog signal
conditioning. If +12V or +15V supplies are present, a simple
+5V regulator can be used. Although it is not suggested, if
the digital supply must be used to power the converter, be
sure to properly filter the supply. Either using a filtered
digital supply or a regulated analog supply, both VDIG and
VANA should be tied to the same +5V source.
INTERMEDIATE LATCHES
The ADS7820 does have tri-state outputs for the parallel
port, but intermediate latches should be used if the bus will
be active during conversions. If the bus is not active during
conversion, the tri-state outputs can be used to isolate the
A/D from other peripherals on the same bus. Tri-state
outputs can also be used when the A/D is the only peripheral
on the data bus.
Intermediate latches are beneficial on any monolithic A/D
converter. The ADS7820 has an internal LSB size of 610µV.
Transients from fast switching signals on the parallel port,
even when the A/D is tri-stated, can be coupled through the
substrate to the analog circuitry causing degradation of
converter performance. The effects of this phenomenon will
be more obvious when using the pin-compatible ADS7821
or any of the other 16-bit converters in the ADS Family. This
is due to the smaller internal LSB size of 38µV.
GROUNDING
Three ground pins are present on the ADS7820. DGND is
the digital supply ground. AGND2 is the analog supply
ground. AGND1 is the ground which all analog signals
internal to the A/D are referenced. AGND1 is more susceptible to current induced voltage drops and must have the path
of least resistance back to the power supply.
All the ground pins of the A/D should be tied to the analog
ground plane, separated from the system’s digital logic
ground, to achieve optimum performance. Both analog and
digital ground planes should be tied to the “system” ground
as near to the power supplies as possible. This helps to
prevent dynamic digital ground currents from modulating
the analog ground through a common impedance to power
ground.
®
ADS7820
10