ADS7800
SBAS001A – OCTOBER 1989 – REVISED FEBRUARY 2004
12-Bit 3µs Sampling
ANALOG-TO-DIGITAL CONVERTER
FEATURES
DESCRIPTION
● 333k SAMPLES PER SECOND
● STANDARD ±10V AND ±5V INPUT RANGES
● DC PERFORMANCE OVER TEMP:
No Missing Codes
1/2LSB Integral Linearity Error
3/4LSB Differential Linearity Error
● AC PERFORMANCE OVER TEMP:
72dB Signal-to-Noise Ratio
80dB Spurious-Free Dynamic Range
–80dB Total Harmonic Distortion
● INTERNAL SAMPLE/HOLD, REFERENCE,
CLOCK, AND THREE-STATE OUTPUTS
● POWER DISSIPATION: 215mW max
● PACKAGE: 24-Pin Single-Wide DIP
24-Lead SOIC
The ADS7800 is a complete 12-bit sampling analog-todigital (A/D) converter using state-of-the-art CMOS
structures. It contains a complete 12-bit successive
approximation A/D converter with internal sample/hold,
reference, clock, digital interface for microprocessor
control, and three-state output drivers.
The ADS7800 is specified at a 333kHz sampling rate.
Conversion time is factory set for 2.70µs max over
temperature, and the high-speed sampling input stage
insures a total acquisition and conversion time of 3µs
max over temperature. Precision, laser-trimmed scaling
resistors provide industry-standard input ranges of ±5V
or ±10V.
AC and DC performance are completely specified. Two
grades based on linearity and dynamic performance are
available to provide the optimum price/performance fit in
a wide range of applications.
The 24-pin ADS7800 is available in plastic and sidebraze hermetic 0.3" wide DIPs, and in an SOIC package.
It operates from a +5V supply and either a –12V or –15V
supply. The ADS7800 is available in grades specified
over 0°C to +70°C and –40°C to +85°C temperature
ranges.
Control
Logic
±10VIN
Clock
Output
Latches
And
Three
State
Drivers
CDAC
±5VIN
2V
Reference
Out
BUSY
SAR
Internal
Ref
Comparator
Three
State
Parallel
Output
Data
Bus
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
All trademarks are the property of their respective owners.
Copyright © 1989-2004, Texas Instruments Incorporated
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of Texas Instruments
standard warranty. Production processing does not necessarily include
testing of all parameters.
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SPECIFICATIONS
ELECTRICAL
At TA = TMIN to TMAX, Sampling Frequency, fS, = 333kHz, –VS = –15V, VS = +5V, unless otherwise specified.
ADS7800JP/JU/AH
PARAMETER
CONDITIONS
MIN
TYP
RESOLUTION
±10V Range
±5V Range
4.4
2.9
±10V/±5V
6.3
4.2
333
2.5
2.6
380
Conversion Alone
Acquisition + Conversion
DC ACCURACY
Full Scale Error (1)
Full Scale Error Drift
Integral Linearity Error
Differential Linearity Error
No Missing Codes
Bipolar Zero(1)
Bipolar Zero Drift
Power Supply Sensitivity
*
*
*
*
*
*
*
2.7
3.0
fIN1
fIN2
67
68
77
–74
–74
70
71
2.0
10
–0.3
+2.4
–5
+5
ISINK = 1.6mA
ISOURCE = 500µA
0.0
+2.4
±0.1
–11.4
+4.75
V
kΩ
kΩ
*
*
µs
µs
kHz
±0.35
%
ppm/°C
LSB(2)
LSB
±2
LSB
ppm/°C
*
*
±1/2
LSB
LSB
LSB
LSB
*
69
70
13
150
130
150
1.9
*
*
*
0.1
77
–77
–77
Bits
Ensured
±1/2
±1/2
±1
74
UNITS
*
±1/2
±3/4
1
fIN = 47kHz
fIN = 47kHz
= 24.4kHz (–6dB)
= 28.5kHz (–6dB)
fIN = 47kHz
fIN = 47kHz
MAX
*
±4
DIGITAL INPUTS
Logic Levels
VIL
VIH
IIL
IIH
2
*
*
Ensured
INTERNAL REFERENCE VOLTAGE
Voltage
Source Current Available
for External Loads
POWER SUPPLIES
Rated Voltage
–VS
VS (VSA and VSD)
Current
–IS
IS
Power Consumption
8.1
5.4
±1
±1
SAMPLING DYNAMICS
Aperture Delay
Aperture Jitter
Transient Response (5)
Overvoltage Recovery (6)
DIGITAL OUTPUTS
Data Format
Data Coding
VOL
VOH
ILEAKAGE (High-Z State)
TYP
±0.50
Transition Noise(3)
Signal-to-(Noise + Distortion) Ratio
Signal-to-Noise Ratio (SNR)
MIN
6
–16.5V < –VS < –13.5V
–12.6V < –VS < –11.4V
+4.75V < VS < +5.25V
AC ACCURACY
Spurious-Free Dynamic Range
Total Harmonic Distortion
Two-tone Intermodulation Distortion
MAX
12
ANALOG INPUT
Voltage Ranges
Impedance
THROUGHPUT SPEED
Conversion Time
Complete Cycle
Throughput Rate
ADS7800KP/KU/BH
2.1
*
+0.8
+5.3
*
*
*
*
Parallel, 12-bit or 8-bit/4-bit
Binary Offset Binary
+0.4
*
+5.0
*
±5
–15
+5.0
–16.5
+5.25
3.5
18
135
6
25
215
*
*
80
–80
–80
–77
–77
dB (4)
dB
dB
72
73
dB
dB
*
*
*
*
ns
ps, rms
ns
ns
*
*
*
V
µA
*
*
V
V
µA
µA
*
*
*
*
V
V
µA
*
*
*
*
V
V
*
*
*
*
*
*
mA
mA
mW
ADS7800
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SBAS001A
SPECIFICATIONS
(CONT)
ELECTRICAL
At TA = TMIN to TMAX, Sampling Frequency, fS, = 333kHz, –VS = –15V, VS = +5V, unless otherwise specified.
ADS7800JP/JU/AH
PARAMETER
TEMPERATURE RANGE
Specification
Operating
Storage
CONDITIONS
MIN
JP/JU/KP/KU
AH/BH
JP/KP/JU/KU
0
–40
–40
–65
TYP
ADS7800KP/KU/BH
MAX
MIN
+70
+85
+85
+150
*
*
*
*
TYP
MAX
UNITS
*
*
*
*
°C
°C
°C
°C
* Same as specification for ADS7800JP/JU/AH.
NOTES: (1) Adjustable to zero with external potentiometer. (2) LSB means Least Significant Bit. For ADS7800, 1LSB = 2.44mV for the ±5V range, 1LSB =
4.88mV for the ±10V range. (3) Noise was characterized over temperature near full scale, 0V, and negative full scale. 0.1LSB represents a typical rms level of
noise at the worst case, which was near full scale input at +125°C. (4) All specifications in dB are referred to a full-scale input, either ±10V or ±5V. (5) For full
scale step input, 12-bit accuracy attained in specified time. (6) Recovers to specified performance in specified time after 2 x FS input overvoltage.
ELECTROSTATIC
DISCHARGE SENSITIVITY
ABSOLUTE MAXIMUM RATINGS
–VS to ANALOG COMMON ............................................................ –16.5V
VS to DIGITAL COMMON .................................................................... +7V
Pin 23 (VSD ) to Pin 24 (VSA ) ........................................................... ±0.3V
ANALOG COMMON to DIGITAL COMMON ....................................... ±1V
Control Inputs to DIGITAL COMMON ............................. –0.3 to VS + 0.3V
Analog Input Voltage .......................................................................... ±20V
Maximum Junction Temperature ..................................................... 160°C
Internal Power Dissipation ............................................................. 750mW
Lead Temperature (soldering, 10s) ............................................... +300°C
Thermal Resistance, θJA:
Plastic DIP ................................................................................ 100°C/W
SOIC ......................................................................................... 100°C/W
Ceramic ...................................................................................... 50°C/W
The ADS7800 is an ESD (electrostatic discharge) sensitive
device. The digital control inputs have a special FET structure, which turns on when the input exceeds the supply by
18V, to minimize ESD damage. However, permanent damage
may occur on unconnected devices subject to high energy
electrostatic fields. When not in use, devices must be stored in
conductive foam or shunts. The protective foam should be
discharged to the destination socket before devices are removed.
PACKAGE/ORDERING INFORMATION
For the most current package and ordering information, see
the Package Option Addendum located at the end of this data
sheet.
ADS7800
SBAS001A
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3
PIN ASSIGNMENTS
PIN # NAME
1
4
PIN CONFIGURATION
Top View
DESCRIPTION
IN1
±10V Analog Input. Connected to GND for ±5V range.
DIP/SOIC
2
IN2
±5V Analog Input. Connected to GND for ±10V range.
3
REF
+2V Reference Output. Bypass to GND with 22µF to
47µF Tantalum. Buffer for external loads.
IN1
1
24
VSA
4
AGND
Analog Ground. Connect to pin 13.
IN2
2
23
VSD
5
D11
Data Bit 11. Most Significant Bit (MSB).
6
D10
Data Bit 10.
REF
3
22
–VS
7
D9
Data Bit 9.
AGND
4
21
BUSY
8
D8
Data Bit 8.
9
D7
Data Bit 7 if HBE is LOW; LOW if HBE is HIGH.
D11
5
20
CS
10
D6
Data Bit 6 if HBE is LOW; LOW if HBE is HIGH.
D10
6
19
R/C
11
D5
Data Bit 5 if HBE is LOW; LOW if HBE is HIGH.
D9
7
18
HBE
12
D4
Data Bit 4 if HBE is LOW; LOW if HBE is HIGH.
13
DGND
Digital Ground. Connect to pin 4.
D8
8
17
D0
14
D3
Data Bit 3 if HBE is LOW; Data Bit 11 if HBE is HIGH.
D7
9
16
D1
15
D2
Data Bit 2 if HBE is LOW; Data Bit 10 if HBE is HIGH.
16
D1
Data Bit 1 if HBE is LOW; Data Bit 9 if HBE is HIGH.
D6
10
15
D2
D5
11
14
D3
D4
12
13
DGND
17
D0
Data Bit 0 if HBE is LOW. Least Significant Bit (LSB);
Data Bit 8 if HBE is HIGH.
18
HBE
High Byte Enable. When held LOW, data output as 12
bits in parallel. When held HIGH, four MSBs presented on
pins 14-17, pins 9-12 output LOWs. Must be LOW to
initiate conversion.
19
R/C
Read/Convert. Falling edge initiates conversion when CS
is LOW, HBE is LOW, and BUSY is HIGH.
20
CS
Chip Select. Outputs in Hi-Z state when HIGH. Must be
LOW to initiate conversion or read data.
21
BUSY
Busy. Output LOW during conversion. Data valid on rising
edge in Convert Mode.
22
–VS
Negative Power Supply. –12V or –15V. Bypass to GND.
23
VSD
Positive Digital Power Supply. +5V. Connect to pin 24,
and bypass to GND.
24
VSA
Positive Analog Power Supply. +5V. Connect to pin 23,
and bypass to GND.
ADS7800
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SBAS001A
TYPICAL PERFORMANCE CURVES
At +VS = +5V, –VS = –15V, and TA = +25°C, unless otherwise noted. All plots use 1024 point FFTs.
FREQUENCY SPECTRUM (10kHz fIN )
FREQUENCY SPECTRUM (50kHz fIN )
0
0
fIN = 10kHz
fSAMPLING = 330kHz
TA = 25°C
fIN = 50kHz
fSAMPLING = 330kHz
TA = 25°C
–20
Magnitude (dB)
Magnitude (dB)
–20
–40
–60
–80
–40
–60
–80
–100
–100
–120
–120
50
0
100
Frequency (kHz)
150 165
0
SIGNAL/(NOISE + DISTORTION) vs
INPUT FREQUENCY AND AMBIENT TEMPERATURE
150 165
Spurious Free Dynamic Range (dB)
95
–55°C
70
C
5°
+2
+1
25
°C
Signal/(Noise + Distortion) (dB)
100
Frequency (kHz)
SPURIOUS FREE DYNAMIC RANGE vs
INPUT FREQUENCY AND AMBIENT TEMPERATURE
75
65
90
85
–5
5°C
+2
5°
+1 C
25
°C
80
75
70
65
1
10
50
150
1
10
50
150
Input Frequency (kHz)
Input Frequency (kHz)
SIGNAL/(NOISE + DISTORTION) vs
FREQUENCY AND AMPLITUDE
SPURIOUS FREE DYNAMIC RANGE vs
INPUT FREQUENCY AND NEGATIVE SUPPLY VOLTAGE
95
Spurious Free Dynamic Range (dB)
80
Signal/(Noise + Distortion) (dB)
50
0dB
60
–20dB
40
–40dB
20
–60dB
90
–V S = –15V
–V S = –12V
85
80
75
70
65
0
1
10
50
150
ADS7800
SBAS001A
1
10
50
150
Input Frequency (kHz)
Input Frequency (kHz)
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5
THEORY OF OPERATION
+5V
The ADS7800 combines the advantages of advanced CMOS
technology (logic density, stable capacitors, and good
analog switches) with Burr-Brown’s proven skills in lasertrimmed thin-film resistors to provide a complete sampling
A/D converter.
47µF
A basic charge-redistribution successive approximation
architecture converts analog input voltages into digital
words. Figure 1 shows the operation of a simplified 3-bit
charge redistribution A/D. Precision laser-trimmed scaling
resistors at the input divide standard input ranges (±10V or
±5V for the ADS7800) into levels compatible with the
CMOS characteristics of the internal capacitor array.
While in the sampling mode, the capacitor array switch for
the MSB capacitor (S1) is in position “S”, so that the charge
on the MSB capacitor is proportional to the voltage level of
the analog input signal, and the remaining array switches (S2
and S3) are set to position “R” to provide an accurate bipolar
offset from the reference source REF. At the same time,
switch SC is also in the closed position to auto-zero any
offset errors in the CMOS comparator.
When a convert command is received, switch S1 is opened
to trap a charge on the MSB capacitor proportional to the
input level at the time of the sampling command, switches
S2 and S3 are opened to trap an offset charge, and switch
SC is opened to float the comparator input. The charge
trapped on the capacitor array can now be moved between
the three capacitors in the array by connecting switches S1,
S2 and S3 to positions “R” (to connect to REF) or “G” (to
connect to GND) successively, changing the voltage generated at the comparator input node.
The first approximation connects the MSB capacitor via
switch S1 to REF, while switches S2 and S3 are connected
to GND. Depending on whether the comparator output is
HIGH or LOW, the logic will then latch S1 in position “R”
or “G”, and moves on to make the next approximation by
connecting S2 to REF and S3 to GND. When the three
successive approximation steps are made for this simple
converter, the voltage level at the comparator will be within
1/2LSB of GND, and the data output word will be based on
reading the positions of S1, S2 and S3.
Signal
Comparator
SC
Input
4C
S
2C
S1
C
S2
S3
To Switches
R
G
R
G
R
G
+
Ref
–
FIGURE 1. 3-Bit Charge Redistribution A/D.
6
L
o
g
i
c
Out
1
IN 1
+5V
24
2
IN 2
+5V
23
3
REF
–15V
22
4
AGND
BUSY
21
5
D11 (MSB) CS
20
6
D10
R/C
19
7
D9
HBE
18
8
D8
D0 (LSB)
17
9
D7
D1
16
10 D6
D2
15
11 D5
D3
14
12 D4
DGND
13
Input
+
+
6.8µF
1µF
+
0.1µF
–15V
Busy
D11
(MSB)
Convert
Command
D0
(LSB)
Data Out
FIGURE 2. Basic ±10V Operation.
OPERATION
BASIC OPERATION
Figure 2 shows the simple hookup circuit required to operate
the ADS7800 in a ±10V range in the Convert Mode. A
convert command arriving on pin 19, R/C, (a pulse taking
pin 19 LOW for a minimum of 40ns) puts the ADS7800 in
the hold mode, and a conversion is started. Pin 21, BUSY,
will be held LOW during the conversion, and rises only after
the conversion is completed and the data has been transferred to the output latches. Thus, the rising edge of the
signal on pin 21 can be used to read the data from the
conversion. Also, during conversion, the BUSY signal puts
the output data lines in Hi-Z states and inhibits input lines.
This means that pulses on pin 19 are ignored, so that new
conversions cannot be initiated during a conversion, either
as a result of spurious signals or to short-cycle the
ADS7800.
In the Read Mode, the input to pin 19 is kept normally LOW,
and a HIGH pulse is used to read data and initiate a
conversion. In this mode, the rising edge of R/C on pin 19
will enable the output data pins, and the data from the
previous conversion becomes valid. The falling edge then
puts the ADS7800 in a hold mode, and initiates a new
conversion.
The ADS7800 will begin acquiring a new sample as soon
as the conversion is completed, even before the BUSY
output rises on pin 21, and will track the input signal until
the next conversion is started, whether in the Convert Mode
or the Read Mode.
ADS7800
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SBAS001A
R/C
tB
BUSY
tDBC
tC
Converter Acquisition
Mode
tAP
Conversion
Acquisition
Conversion
FIGURE 3. Acquisition and Conversion Timing.
PARAMETER
tDBC
tB
tAP
∆tAP
tC
BUSY delay from R/C
BUSY Low
Aperture Delay
Aperture Jitter
Conversion Time
R/C
1
0
0
X
1↓0
1
HBE BUSY
X
0
0
1
1
1
0
0
0
X
1
1↓0
0
X
1
1
1
X
1
1
1
0
OPERATION
None - Outputs in Hi-Z State.
Holds Signal and Initiates Conversion.
Output Three-State Buffers Enabled once
Conversion has Finished.
Enable Hi-Byte in 8-bit Bus Mode.
Inhibit Start of Conversion.
None - Outputs in Hi-Z State.
Conversion in Progress. Outputs Hi-Z
State. New Conversion Inhibited until
Present Conversion has Finished.
TABLE II. Control Line Functions.
Hold Time
SYMBOL
CS
MIN
TYP
MAX
UNITS
80
2.5
13
150
2.47
150
2.7
ns
µs
ns
ps, rms
µs
2.70
For stand-alone operation, control of the ADS7800 is
accomplished by a single control line connected to R/C. In
this mode, CS and HBE are connected to GND. The output
data are presented as 12-bit words. The stand-alone mode
is used in systems containing dedicated input ports which
do not require full bus interface capability.
TABLE I. Acquisition and Conversion Timing.
For use with an 8-bit bus, the data can be read out in two
bytes under the control of pin 18, HBE. With a LOW input
on pin 18, at the end of a conversion, the 8 LSBs of data
are loaded into the latches on pins 9 through 12 and 14
through 17. Taking pin 18 HIGH then loads the 4 MSBs on
pins 14 through 17, with pins 9 through 12 being forced
LOW.
ANALOG INPUT RANGES
The ADS7800 offers two standard bipolar input ranges:
±10V and ±5V. If a ±10V range is required, the analog input
signal should be connected to pin 1. A signal requiring a
±5V range should be connected to pin 2. In either case, the
other pin of the two must be grounded or connected to the
adjustment circuits described in the section on calibration.
(See Figures 4 and 5, or 10 and 11.)
Conversion is initiated by a HIGH-to-LOW transition on
R/C. The three-state data output buffers are enabled when
R/C is HIGH and BUSY is HIGH. Thus, there are two
possible modes of operation: conversion can be initiated
with either positive or negative pulses. In either case, the
R/C pulse must remain LOW a minimum of 40ns.
Figure 6 illustrates timing when conversion is initiated by
an R/C pulse which goes LOW and returns HIGH during the
conversion. In this case (Convert Mode), the three-state
outputs go into the Hi-Z state in response to the falling edge
of R/C, and are enabled for external access of the data after
completion of the conversion.
Figure 7 illustrates the timing when conversion is initiated
by a positive R/C pulse. In this mode (Read Mode), the
output data from the previous conversion is enabled during
the HIGH portion of R/C. A new conversion starts on the
falling edge of R/C, and the three-state outputs return to the
Hi-Z state until the next occurrence of a HIGH on R/C.
CONVERSION START
CONTROLLING THE ADS7800
The ADS7800 can be easily interfaced to most microprocessor-based and other digital systems. The microprocessor
may take full control of each conversion, or the ADS7800
may operate in a stand-alone mode, controlled only by the
R/C input. Full control consists of initiating the conversion
and reading the output data at user command, transmitting
data either all 12-bits in one parallel word, or in two 8-bit
bytes. The three control inputs (CS, R/C and HBE) are all
TTL/CMOS compatible. The functions of the control lines
are shown in Table II.
A conversion is initiated on the ADS7800 only by a negative
transition occurring on R/C, as shown in Table I. No other
combination of states or transitions will initiate a conversion. Conversion is inhibited if either CS or HBE are HIGH,
or if BUSY is LOW. CS and HBE should be stable a
minimum of 25ns prior to the transition on R/C. Timing
relationships for start of conversion are illustrated in Figure
8.
The BUSY output indicates the current state of the converter
by being LOW only during conversion. During this time the
three-state output buffers remain in a Hi-Z state, and
therefore data cannot be read during conversion. During this
period, additional transitions on the three digital inputs (CS,
R/C and HBE) will be ignored, so that conversion cannot
be prematurely terminated or restarted.
ADS7800
SBAS001A
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7
INTERNAL CLOCK
The ADS7800 has an internal clock that is factory trimmed
to achieve a typical conversion time of 2.47µs, and a
maximum conversion time over the full operating temperature range of 2.7µs. No external adjustments are required,
and with the guaranteed maximum acquisition time of
300ns, throughput performance is assured with convert
pulses as close as 3µs.
1
ADS7800
±5V
Input
2
FIGURE 5. ±5V Range Without Trims.
READING DATA
After conversion is initiated, the output buffers remain in a
Hi-Z state until the following three logic conditions are
simultaneously met: R/C is HIGH, BUSY is HIGH and CS
is LOW. Upon satisfaction of these conditions, the data lines
are enabled according to the state of HBE. See Figure 9 and
Table III for timing relationships and specifications.
CALIBRATION PROCEDURE
First, trim offset, by applying at the input (pin 1 or 2) the
mid-point transition voltage (–2.44mV for the ±10V range,
–1.22mV for the ±5V range.) With the ADS7800 converting
continually, adjust potentiometer R1 until the MSB (D11 on
pin 5) is toggling alternately HIGH and LOW.
CALIBRATION
Next adjust full scale, by applying at the input a DC input
signal that is 3/2LSB below the nominal full scale voltage
(+9.9927V for the ±10V range, +4.9963V for the ±5V
range.) With the ADS7800 converting continually, adjust
R2 until the LSB (D0 on pin 17) is toggling HIGH and LOW
with all of the other bits HIGH.
OPTIONAL EXTERNAL GAIN AND OFFSET TRIM
Offset and full-scale errors may be trimmed to zero using
external offset and full-scale trim potentiometers connected
to the ADS7800 as shown in Figures 10 and 11.
LAYOUT CONSIDERATIONS
If adjustment of offset and full scale is not required,
connections as shown in Figures 4 and 5 should be used.
±10V
Input
Because of the high resolution and linearity of the ADS7800,
system design problems such as ground path resistance and
contact resistance become very important.
ANALOG SIGNAL SOURCE IMPEDANCE
The input resistance of the ADS7800 is 6.3kΩ or 4.2kΩ (for
the ±10V and ±5V ranges respectively.) To avoid introducing distortion, the source resistance must be very low, or
constant with signal level. The output impedance provided
by most op amps is ideal.
1
ADS7800
2
Pins 23 (VSD ) and 24 (VSA ) are not connected internally
on the ADS7800, to maximize accuracy on the chip. They
should be connected together as close as possible to the unit.
FIGURE 4. ±10V Range Without Trims.
tW
R/C
tB
BUSY
tDBC
tAP
Converter
Mode
Acquire
tDBE
Convert
Acquire
tC
tA
tDB
tHDR and tHL
Data
BUS
Data Valid
Convert
Hi-Z State
Data Valid
Hi-Z State
FIGURE 6. Convert Mode: R/C Pulse LOW — Outputs Enabled After Conversion.
8
ADS7800
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SBAS001A
R/C
tW
tB
BUSY
tDBC
tAP
Converter
Mode
Acquire
tDBE
tAP
Convert
Acquire
tC
tA
tDD
Convert
tHDR and tHL
Data
BUS
Hi-Z State
Data
Valid
Hi-Z State
Data
Valid
Hi-Z State
FIGURE 7. Read Mode: R/C Pulse HIGH— Outputs Enabled Only When R/C is High.
SYMBOL
tW
tDBC
PARAMETER
MIN
TYP
R/C Pulse Width
40
10
MAX
UNITS
ns
BUSY delay from R/C
80
150
ns
tB
BUSY LOW
2.5
2.7
µs
tAP
Aperture Delay
13
ns
∆tAP
Aperture Jitter
150
ps, rms
tC
2.70
µs
75
200
ns
Conversion Time
2.47
tDBE
BUSY from End of Conversion
100
tDB
BUSY Delay after Data Valid
25
ns
tA
Acquisition Time
130
300
ns
tA+tC
Throughput Time
2.6
3.0
µs
tHDR
tS
Valid Data Held After R/C LOW
20
50
ns
CS or HBE LOW before R/C Falls
25
5
ns
25
tH
CS or HBE LOW after R/C Falls
tDD
Data Valid from CS LOW, R/C HIGH, and HBE in Desired State (Load = 100pF)
tHDR
Valid Data Held After R/C Low
tHL
20
Delay to Hi-Z State after R/C Falls or CS Rises (3kΩ Pullup or Pulldown)
0
65
ns
150
50
50
ns
ns
150
ns
TABLE III. Timing Specifications (TMIN to TMAX).
tS
CS or
HBE
tH
To limit the effects of digital switching elsewhere in a
system on the analog performance of the system, it often
makes sense to run a separate +5V supply conductor from
the supply regulator to any analog components requiring
+5V, including the ADS7800.
tW
R/C
tDBC
BUSY
Data
Bus
Pin 24 may be slightly more sensitive than pin 23 to supply
variations, but to maintain maximum system accuracy, both
should be well isolated from digital supplies with wide load
variations.
Data Valid
Hi-Z State
tHDR and tHL
FIGURE 8. Conversion Start Timing.
The VS pins (23 and 24) should be connected together and
bypassed with a parallel combination of a 6.8µF tantalum
capacitor and a 0.1µF ceramic capacitor located close to the
converter to obtain noise-free operation. (See Figure 2.) The
–VS pin 22 should be bypassed with a 1µF tantalum
capacitor, again as close as possible to the ADS7800.
Noise on the power supply lines can degrade converter
performance, especially noise and spikes from a switching
power supply. Appropriate supplies or filters must be used.
The GND pins (4 and 13) are also separated internally, and
should be directly connected to a ground plane under the
ADS7800
SBAS001A
www.ti.com
9
converter if at all possible. A ground plane is usually the best
solution for preserving dynamic performance and reducing
noise coupling into sensitive converter circuits. Where any
compromises must be made, the common return of the
analog input signal should be referenced to pin 4, AGND,
on the ADS7800, which prevents any voltage drops that
might occur in the power supply common returns from
appearing in series with the input signal.
Coupling between analog input and digital lines should be
minimized by careful layout. For instance, if the lines must
cross, they should do so at right angles. Parallel analog and
digital lines should be separated from each other by a pattern
connected to common.
If external full scale and offset potentiometers are used, the
potentiometers and related resistors should be located as
close to the ADS7800 as possible.
CS
R/C
HBE
BUSY
tDB
DB11-DB0
Data Valid
tDD
tHL & tHDR
FIGURE 9. Read Cycle Timing.
REFERENCE BYPASS
Pin 3 (REF) should be bypassed with a 22µF to 47µF
tantalum capacitor. A rated working voltage of 2V or more
is acceptable here. This pin is used to enhance the system
accuracy of the internal reference circuit, and is not
recommended for driving external signals. If there are
important system reasons for using the ADS7800 reference
externally, the output of pin 3 must be appropriately
buffered.
“HOT SOCKET” PRECAUTION
Two separate +5V VS pins, 23 and 24, are used to minimize
noise caused by digital transients. If one pin is powered and
the other is not, the ADS7800 may “Latch Up” and draw
excessive current. In normal operation, this is not a problem
because both pins will be soldered together. However,
during evaluation, incoming inspection, repair, etc., where
the potential of a “Hot Socket” exists, care should be taken
to power the ADS7800 only after it has been socketed.
10
External
Gain Adjust
±10V
Input
R2
1
ADS7800
100Ω
2
3
+5V
Bipolar
Zero
Adjust
R1
4
10kΩ
10k
49.9Ω
6.65kΩ
5
6
–15V
7
FIGURE 10. ±10V Range With External Trims.
MINIMIZING “GLITCHES”
Coupling of external transients into an A/D converter can
cause errors which are difficult to debug. In addition to the
discussions earlier on layout considerations for supplies,
bypassing and grounding, there are several other useful
steps that can be taken to get the best analog performance
out of a system using the ADS7800. These potential system
problem sources are particularly important to consider when
developing a new system, and looking for the causes of
errors in breadboards.
First, care should be taken to avoid glitches during critical
times in the sampling and conversion process. Since the
ADS7800 has an internal sample/hold function, the signal
that puts it into the hold state (R/C going LOW) is critical, as
it would be on any sample/hold amplifier. The R/C falling
edge should be sharp and have minimal ringing, especially
during the 20ns after it falls.
Although not normally required, it is also good practice to
avoid glitching the ADS7800 while bit decisions are being
made. Since the above discussion calls for a fast, clean rise
and fall on R/C, it makes sense to keep the rising edge of the
convert pulse outside the time when bit decisions are being
made. In other words, the convert pulse should either be
short (under 100ns so that it transitions before the MSB
decision), or relatively long (over 2.75µs to transition after
the LSB decision).
External
Gain Adjust
±5V
Input
1
R2
2
100Ω
3
+5V
Bipolar
Zero
Adjust
4
R1
10kΩ
10kΩ
ADS7800
5
30.1kΩ
301Ω
6
7
–15V
FIGURE 11. ±5V Range With External Trims.
ADS7800
www.ti.com
SBAS001A
Next, although the data outputs are forced into a Hi-Z state
during conversion, fast bus transients can still be capacitively coupled into the ADS7800. If the data bus experiences
fast transients during conversion, these transients can be
attenuated by adding a logic buffer to the data outputs. The
BUSY output can be used to enable the buffer.
Naturally, transients on the analog input signal are to be
avoided, especially at times within ±20ns of R/C going
LOW, when they may be trapped as part of the charge on the
capacitor array. This requires careful layout of the circuit in
front of the ADS7800.
Finally, in multiplexed systems, the timing on when the
multiplexer is switched may affect the analog performance
of the system. In most applications, the multiplexer can be
switched as soon as R/C goes LOW (with appropriate
delays), but this may affect the conversion if the switched
signal shows glitches or significant ringing at the ADS7800
input. Whenever possible, it is safer to wait until the
conversion is completed before switching the multiplexer.
The extremely fast acquisition time and conversion time of
the ADS7800 make this practical in many applications.
INPUT VOLTAGE RANGE AND LSB VALUES
Input Voltage Range Defined As:
Analog Input Connected to Pin
Pin Connected to GND
One Least Significant Bit (LSB)
FSR/212
±10V
1
2
20V/212
±5V
2
1
10V/212
4.88mV
2.44mV
+10V–3/2LSB
+9.9927V
0V–1/2LSB
–2.44mV
–10V+1/2LSB
–9.9976V
+5V–3/2LSB
+4.9963V
0V–1/2LSB
–1.22mV
–5V+1/2LSB
–4.9988V
OUTPUT TRANSITION VALUES
FFEH to FFFH
+Full Scale
7FFH to 800H
Mid Scale
(Bipolar Zero)
–Full Scale
000H to 001H
TABLE IV. Input Voltages, Transition Values, and LSB Values.
ADS7800
SBAS001A
www.ti.com
11
PACKAGE OPTION ADDENDUM
www.ti.com
7-Oct-2021
PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package Pins Package
Drawing
Qty
Eco Plan
(2)
Lead finish/
Ball material
MSL Peak Temp
Op Temp (°C)
Device Marking
(3)
(4/5)
(6)
ADS7800AH
NRND
CDIP SB
JDN
24
1
RoHS & Green
Call TI
N / A for Pkg Type
ADS7800BH
NRND
CDIP SB
JDN
24
1
RoHS & Green
Call TI
N / A for Pkg Type
ADS7800AH
ADS7800JU
ACTIVE
SOIC
DW
24
25
RoHS & Green
Call TI
Level-3-260C-168 HR
-40 to 85
ADS7800JU
ADS7800JU/1K
ACTIVE
SOIC
DW
24
1000
RoHS & Green
Call TI
Level-3-260C-168 HR
-40 to 85
ADS7800JU
ADS7800JU/1KE4
ACTIVE
SOIC
DW
24
1000
RoHS & Green
Call TI
Level-3-260C-168 HR
-40 to 85
ADS7800JU
ADS7800KU
ACTIVE
SOIC
DW
24
25
RoHS & Green
Call TI
Level-3-260C-168 HR
-40 to 85
ADS7800KU
ADS7800KUE4
ACTIVE
SOIC
DW
24
25
RoHS & Green
Call TI
Level-3-260C-168 HR
-40 to 85
ADS7800KU
ADS7800BH
(1)
The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2)
RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance
do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may
reference these types of products as "Pb-Free".
RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption.
Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of
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