BB ADS7891

SLAS410 − DECEMBER 2003
FEATURES
D 3 MHz Sample Rate, 14-Bit Resolution
D Zero Latency
D Unipolar, Pseudo Differential Input, Range:
D
D
D
D
D
D
D
D
D
− 0 V to 2.5 V
High Speed Parallel Interface
78 dB SNR and 88.5 dB THD at 3 MSPS
Power Dissipation 85 mW at 3 MSPS
Nap Mode (10 mW Power Dissipation)
Power Down (10 mW)
Internal Reference
Internal Reference Buffer
8-/14-Bit Bus Transfer
48-Pin TQFP Package
APPLICATIONS
D Optical Networking (DWDM, MEMS Based
Switching)
Spectrum Analyzers
High Speed Data Acquisition Systems
D
D
D High Speed Close-Loop Systems
D Telecommunication
D Ultra-Sound Detection
DESCRIPTION
The ADS7891 is a 14-bit 3-MSPS A-to-D converter with 2.5-V internal reference. The device includes a
capacitor based SAR A/D converter with inherent sample and hold. The device offers a 14-bit parallel interface
with an additional byte mode that provides easy interface with 8-bit processors. The device has a
pseudo-differential input stage.
The −IN swing of ±200 mV is useful to compensate for ground voltage mismatch between the ADC and sensor
and also to cancel common-mode noise. With nap mode enabled, the device operates at lower power when
used at lower conversion rates. The device is available in a 48-pin TQFP package.
BYTE
SAR
+IN
−IN
+
_
Output
Latches
and
3-State
Drivers
CDAC
14/8-Bit Parallel
Data Output Bus
Comparator
REFIN
CLOCK
REFOUT
2.5 V
Internal
Reference
Conversion
and
Control Logic
PWD/RST
CONVST
BUSY
CS
RD
A_PWD
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.
!"# $ %&'# "$ (&)*%"# +"#', +&%#$
%! # $('%%"#$ (' #-' #'!$ '."$ $#&!'#$ $#"+"+ /""#0,
+&%# (%'$$1 +'$ # '%'$$"*0 %*&+' #'$#1 "** (""!'#'$,
Copyright  2003, Texas Instruments Incorporated
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SLAS410 − DECEMBER 2003
This integrated circuit can be damaged by ESD. Texas Instruments recommends that all integrated circuits be handled with appropriate
precautions. Failure to observe proper handling and installation procedures can cause damage.
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 its published specifications.
ORDERING INFORMATION
MODEL
MAXIMUM
INTEGRAL
LINEARITY
(LSB)
ADS7891
MAXIMUM
DIFFERENTIAL
LINEARITY
(LSB)
±1.5
NO MISSING
CODES AT
RESOLUTION
(BIT)
PACKAGE
TYPE
14
48-Pin
TQFP
+1.5/−1
PACKAGE
DESIGNATOR
PFB
TEMPERATURE
RANGE
ORDERING
INFORMATION
TRANSPORT
MEDIA QUANTITY
ADS7891IPFBT
Tape and reel
250
ADS7891IPFBR
Tape and reel
1000
−40°C to 85°C
NOTE: For most current specifications and package information, refer to the TI website at www.ti.com.
ABSOLUTE MAXIMUM RATINGS
over operating free-air temperature range(1)
UNIT
+IN to AGND
−0.3 V to +VA + 0.1 V
−IN to AGND
−0.3 V to 0.5 V
+VA to AGND
−0.3 V to 7 V
+VBD to BDGND
−0.3 V to 7 V
Digital input voltage to GND
−0.3 V to (+VBD + 0.3 V)
Digital output to GND
−0.3 V to (+VBD + 0.3 V)
Operating temperature range
−40°C to 85°C
Storage temperature range
−65°C to 150°C
Junction temperature (TJmax)
150°C
Power dissipation
TQFP package
Lead temperature, soldering
θJA Thermal impedance
Vapor phase (60 sec)
Infrared (15 sec)
(TJ Max–TA)/ θJA
86°C/W
215°C
220°C
(1) Stresses beyond those listed under “absolute maximum ratings” may cause permanent damage to the device. These are stress ratings only, and
functional operation of the device at these or any other conditions beyond those indicated under “recommended operating conditions” is not
implied. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
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SLAS410 − DECEMBER 2003
SPECIFICATIONS
TA = −40°C to 85°C, +VA = 5 V, +VBD = 5 V or 3.3 V, Vref = 2.5 V, fsample = 3 MHz (unless otherwise noted)
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
ANALOG INPUT
Full-scale input span(1)
Absolute input range
+IN – (−IN)
0
+IN
−0.2
−IN
−0.2
Input capacitance
Input leakage current
Vref
Vref + 0.2
+0.2
V
V
27
pF
500
pA
14
Bits
SYSTEM PERFORMANCE
Resolution
No missing codes
Integral linearity(2)
−1.5
14
±0.75
1.5
−1
±0.75
1.5
Bits
LSB(3)
LSB(3)
Differential linearity
Offset error(4)
External reference
−1.5
±0.2
1.5
mV
Gain error(4)
External reference
−1
±0.2
1
mV
Common-mode rejection ratio
With common mode input signal = 200
mVp−p at 1 MHz
60
dB
Power supply rejection
At 3FF0H output code,
+VA = 4.75 V to 5.25 V , Vref = 2.50 V
80
dB
SAMPLING DYNAMICS
Conversion time
Acquisition time
+VDB = 5 V
255
+VDB = 3 V
+VDB = 5 V
60
+VDB = 3 V
60
273
nsec
273
nsec
78
nsec
nsec
Maximum throughput rate
3
MHz
Aperture delay
2
nsec
Aperture jitter
20
psec
Step response
50
nsec
Over voltage recovery
50
nsec
DYNAMIC CHARACTERISTICS
Total harmonic distortion(5)
VIN = 2.496 Vp−p at 100 kHz/2.5 Vref
VIN = 2.496 Vp−p at 1 MHz/2.5 Vref
−93
−88.5
VIN = 2.496 Vp−p at 1.4 MHz/2.5 Vref
VIN = 2.496 Vp−p at 100 kHz/2.5 Vref
−79.5
VIN = 2.496 Vp−p at 1 MHz/2.5 Vref
VIN = 2.496 Vp−p at 1.4 MHz/2.5 Vref
78
VIN = 2.496 Vp−p at 100 kHz/2.5 Vref
VIN = 2.496 Vp−p at 1 MHz/2.5 Vref
78
SINAD
73.8
SFDR
VIN = 2.496 Vp−p at 1.4 MHz/2.5 Vref
VIN = 2.496 Vp−p at 1 MHz/2.5 Vref
SNR
−87
dB
78.5
dB
75
77
88
−3 dB Small signal bandwidth
dB
90
dB
50
MHz
EXTERNAL REFERENCE INPUT
Input VREF range
Resistance(6)
2.4
2.5
500
2.6
V
kΩ
(1) Ideal input span; does not include gain or offset error.
(2) This is endpoint INL, not best fit.
(3) LSB means least significant bit.
(4) Measured relative to actual measured reference.
(5) Calculated on the first nine harmonics of the input frequency.
(6) Can vary ±20%.
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SLAS410 − DECEMBER 2003
SPECIFICATIONS Continued
TA = −40°C to 85°C, +VA = 5 V, +VBD = 5 V or 3.3 V, Vref = 2.5 V, fsample = 3 MHz (unless otherwise noted)
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
INTERNAL REFERENCE OUTPUT
From 95% (+VA), with 1-µF storage
capacitor on REFOUT to AGND
Start-up time
VREF Range
Source current
IOUT=0
Line regulation
+VA = 4.75 V to 5.25 V
Drift
IOUT = 0
2.48
2.5
Static load
120
msec
2.52
V
10
µA
1
mV
25
PPM/C
DIGITAL INPUT/OUTPUT
Logic family
Logic level
CMOS
VIH
VIL
IIH = 5 µA
IIL = 5 µA
VOH
VOL
IOH = 2 TTL loads
IOL = 2 TTL loads
+VBD −1
−0.3
+VBD + 0.3
0.8
V
+VBD − 0.6
0
+VBD
0.4
V
V
V
Straight
Binary
Data format
POWER SUPPLY REQUIREMENTS
Power supply voltage
+VBD
+VA
2.7
3.3
5.25
V
4.75
5
5.25
V
17
18
mA
85
90
mW
2
3
Supply current, +VA, 3 MHz sample rate
Power dissipation, 3 MHz sample rate
+VA = 5 V
NAP MODE
Supply current, +VA
Power-up time(1)
60
mA
nsec
POWER DOWN
Supply current, +VA
Power down time(2)
Power up time
2
2.5
µA
From simulation results
10
µsec
1-µF Storage capacitor on REFOUT to
AGND
25
msec
Invalid conversions after power up or reset
4
Numbers
85
°C
TEMPERATURE RANGE
Operating free-air
−40
(1) Minimum acquisition time for first sampling after the end of nap state must be 60 nsec more than normal.
(2) Time required to reach level of 2.5 µA.
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SLAS410 − DECEMBER 2003
TIMING REQUIREMENTS
All specifications typical at −40°C to 85°C, +VA = +5 V, +VBD = +5 V (see Notes 1, 2, 3, and 4)
PARAMETER
Conversion time
Acquisition time
SYMBOL
t(conv)
t(acq)
MIN
60
TYP
MAX
UNITS
REF FIG.
255
273
ns
5
ns
5
ns
3
ns
1
ns
1
ns
1
ns
1
ns
2
ns
2
ns
2
ns
2
78
SAMPLING AND CONVERSION START
Hold time CS low to CONVST high (with BUSY high)
Delay CONVST high to acquisition start
Hold time, CONVST high to CS high with BUSY low
Hold time, CONVST low to CS high
Delay CONVST low to BUSY high
CS width for acquisition or conversion to start
Delay CS low to acquisition start with CONVST high
Pulse width, from CS low to CONVST low for acquisition to start
Delay CS low to BUSY high with CONVST low
Quiet sampling time(3)
th1
td1
10
th2
th3
10
td2
tw3
td3
tw1
2
4
5
10
40
20
2
4
5
20
td4
40
25
ns
CONVERSION ABORT
Setup time CONVST high to CS low with BUSY high
Delay time CS low to BUSY low with CONVST high
tsu1
td5
15
ns
4
20
ns
4
td6
td7
25
ns
5
25
ns
5
td9
td11
25
ns
5
20
ns
5
t1
td8
25
ns
5
25
ns
6
td10
t2
25
ns
6
25
ns
6
10
DATA READ
Delay RD low to data valid with CS low
Delay BYTE high to LSB word valid with CS and RD low
Delay time RD high to data 3-state with CS low
Delay time end of conversion to BUSY low
Quiet sampling time RD high to CONVST low
Delay CS low to data valid with RD low
Delay CS high to data 3-state with RD low
Quiet sampling time CS low to CONVST low
BACK-TO-BACK CONVERSION
Delay BUSY low to data valid
td12
tw4
ns
7, 8
Pulse width, CONVST high
70
ns
7, 8
Pulse width, CONVST low
tw5
20
ns
7
tw6
tw7
ns
12
7200
ns
11
POWER DOWN/RESET
Pulse width, low for PWD/RST to reset the device
Pulse width, low for PWD/RST to power down the device
45
6140
Delay time, power up after PWD/RST is high
td13
25
ms
11
(1) All input signals are specified with tr = tf = 5 ns (10% to 90% of +VBD) and timed from a voltage level of (VIL + VIH)/2.
(2) See timing diagram.
(3) Quiet period before conversion start, no data bus activity including data bus 3-state is allowed in this period.
(4) All timings are measured with 20 pF equivalent loads with 5 V +VBD and 10-pF equivalent loads with 3 V +VBD on all data bits and BUSY pin.
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SLAS410 − DECEMBER 2003
PWD/RST
A_PWD
BYTE
CONVST
CS
RD
+VA
AGND
AGND
+VA
REFM
REFM
PIN ASSIGNMENTS
48 47 46 45 44 43 42 41 40 39 38 37
REFIN
1
36
BUSY
REFOUT
2
35
BDGND
NC
3
34
+VBD
+VA
4
33
NC
NC
5
32
+IN
6
31
DB0
−IN
7
30
DB1
AGND
8
29
+VA
9
28
DB2
DB3
+VA
10
27
DB4
11
26
DB5
AGND 12
25
BDGND
AGND
6
DB7
DB6
DB8
DB9
DB10
DB11
DB12
DB13
AGND
+VA
NC − No connection
AGND
13 14 15 16 17 18 19 20 21 22 23
24
+VBD
AGND
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SLAS410 − DECEMBER 2003
Terminal Functions
PIN
NAME
16−23,
26−31
DATA BUS
I/O
DESCRIPTION
8 BIT BUS
BYTE =
0
16 BIT BUS
1
0
16
DB13
O
D13 (MSB)
D5
D13 (MSB)
17
DB12
O
D12
D4
D12
18
DB11
O
D11
D3
D11
19
DB10
O
D10
D2
D10
20
DB9
O
D9
D1
D9
21
DB8
O
D8
D0 (LSB)
D8
22
DB7
O
D7
0
D7
23
DB6
O
D6
0
D6
26
DB5
O
D5
0
D5
27
DB4
O
D4
0
D4
28
DB3
O
D3
0
D3
29
DB2
O
D2
0
D2
30
DB1
O
D1
0
D1
31
DB0
O
D0 (LSB)
0
D0 (LSB)
36
BUSY
O
Status output. This pin is high when a conversion is in progress.
39
BYTE
I
Byte select input. Used for 8-bit bus reading.
0: No fold back.
1: Lower byte D[5:0] is folded back to high byte so D5 is available in D13 place.
40
CONVST
I
Conversion start. The rising edge starts the acquisition. The falling edge of this input ends the
acquisition and starts the conversion. Refer to the timing diagrams for more details.
41
RD
I
Active low synchronization pulse for the parallel output. When CS is low, this serves as the output
enable and puts the previous conversion results on the bus.
37
A_PWD
I
Nap mode enable, active low
24, 34
+VBD
Digital power supply for all digital inputs and outputs. Refer to Table 3 for layout guidelines.
25, 35
BDGND
Digital ground for all digital inputs and outputs. Needs to be shorted to analog ground plane below
the device.
42
CS
I
Chip Select. Active low signal enables chip operation like acquisition start, conversion start, bus
release from 3-state. Refer to the timing diagrams for more details.
38
PWD/RST
I
Active low input, acts as device power down/device reset signal.
5, 8, 11, 12, 14,
15, 44, 45
AGND
Analog ground pins. Need to be shorted to analog ground plane below the device.
4, 9, 10, 13, 43,
46
+VA
Analog power supplies. Refer to Table 3 for layout guidelines.
6
+IN
I
Non inverting analog input channel
7
−IN
I
Inverting analog input channel
1
REFIN
I
Reference (positive) input. Needs to be decoupled with REFM pin using 0.1-µF bypass capacitor
and 1-µF storage capacitor.
2
REFOUT
O
Internal reference output. To be shorted to REFIN pin when internal reference is used. Do not
connect to REFIN pin when external reference is used. Always needs to be decoupled with AGND
using 0.1-µF bypass capacitor.
47, 48
REFM
I
Reference ground. To be connected to analog ground plane.
3, 32, 33
NC
No connection pins.
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SLAS410 − DECEMBER 2003
DESCRIPTION AND TIMING DIAGRAMS
SAMPLING AND CONVERSION START
There are three ways to start sampling. The rising edge of CONVST starts sampling with CS and BUSY being low
(see Figure 1) or it can be started with the falling edge of CS when CONVST is high and BUSY is low (see
Figure 2). Sampling can also be started with an internal conversion end (before BUSY falling edge) with CS
being low and CONVST high before an internal conversion end (see Figure 3). Also refer to the section DEVICE
OPERATION AND DATA READ IN BACK-TO-BACK CONVERSION for more details.
A conversion can be started two ways (a conversion start is the end of sampling). Either with the falling edge
of CONVST when CS is low (see Figure 1) or the falling edge of CS when CONVST is low (see Figure 2). A
clean and low jitter falling edge of these respective signals triggers a conversion start and is important to the
performance of the converter. The BUSY pin is brought high immediately following the CONVST falling edge.
BUSY stays high throughout the conversion process and returns low when the conversion has ended.
th2
th3
CS
CONVST
td1
td2
BUSY
t(acq)
Figure 1. Sampling and Conversion Start Control With CONVST Pin
tw3
tw3
CS
td4
CONVST
td3
tw1
BUSY
t(acq)
Figure 2. Sampling and Conversion Start Control With CS Pin
CS
th1
tw5
CONVST
tw4
BUSY
td2
t(acq)
Figure 3. Sampling Start With CS Low and CONVST High (Back-to-Back)
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SLAS410 − DECEMBER 2003
CONVERSION ABORT
The falling edge of CS aborts the conversion while BUSY is high and CONVST is high (see Figure 4). The device
outputs 3F80 (hex) to indicate a conversion abort.
td5
BUSY
tsu1
CONVST
CS
RD
11 1111 1000 0000
D13−D0
Figure 4. Conversion Abort
DATA READ
Two conditions need to be satisfied for a read operation. Data appears on the D13 through D0 pins (with D13
MSB) when both CS and RD are low. Figure 5 and Figure 6 illustrate the device read operation. The bus is
three-stated if any one of the signals is high.
t1
td2
tw5
CONVST
t(conv)
td1 + t(acq)
BUSY
td11
CS
RD
BYTE
td6
D13−D0
td7
D13−6 & D5−0
td9
D5−0
Figure 5. Read Control Via CS and RD
There are two output formats available. Fourteen bit data appears on the bus during a read operation while
BYTE is low. When BYTE is high, the lower byte (D5 through D0 followed by all zeroes) appears on the data
bus with D5 in the MSB. This feature is useful for interfacing with eight bit microprocessors and microcontrollers.
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SLAS410 − DECEMBER 2003
t2
CONVST
td1 + t(acq)
BUSY
Conversion No N
td2
CS
BYTE
td7
D13−6 & D5−0
D13−D0
Data For Conv. N−1
td10
D5−0
Data For Conv. N
td8
Figure 6. Read Control Via CS and RD Tied to BDGND
DEVICE OPERATION AND DATA READ IN BACK-TO-BACK CONVERSION
The following two figures illustrate device operation in back-to-back conversion mode. It is possible to operate
the device at any throughput in this mode, but this is the only mode in which the device can be operated at
throughputs exceeding 2.8 MSPS.
A conversion starts on the CONVST falling edge. The BUSY output goes high after a delay (td2). Note that care
must be taken not to abort the conversion (see Figure 4) apart from timing restrictions shown in Figure 7 and
Figure 8. The conversion ends within the conversion time, t(conv), after the CONVST falling edge. The new
acquisition can be immediately started without waiting for the BUSY signal to go low. This can be ensured with
a CONVST high pulse width that is more than or equal to (t0 – t(conv) + 10 nsec) which is tw4 for a 3-MHz operation.
Sample N
CONVST
tw4
t(acq)
tw5
Conversion N
BUSY
td12
D13−D0
t(conv) + td11
Data For Conversion N−1
(Data read Without Latency)
t0 = 333 ns for 3 MSPS Operation
Figure 7. Back-To-Back Operation With CS and RD Low
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SLAS410 − DECEMBER 2003
CS
Sample N
th1
CONVST
tw4
t(acq)
tw3
t(conv) + td11
Conversion N
BUSY
Data For Conversion N−1
td12
D13−D0
(Data read Without Latency)
t0 = 333 ns for 3 MSPS Operation
Figure 8. Back-To-Back operation With CS Toggling and RD Low
NAP MODE
The device can be put in nap mode following the sequences shown in Figure 9. This provides substantial power
saving while operating at lower sampling rates.
While operating the device at throughput rates lower than 2.54 MSPS, A_PWD can be held low (see Figure 9).
In this condition, the device goes into the nap state immediately after BUSY goes low and remains in that state
until the next sampling starts. The minimum acquisition time is 60 nsec more than t(acq) as defined in the timing
requirements section.
Alternately, A_PWD can be toggled any time during operation (see Figure 10). This is useful when the system
acquires data at the maximum conversion speed for some period of time (back-to-back conversion) and it does
not acquire data for some time while the acquired data is being processed. During this period, the device can
be put in the nap state to save power. The device remains in the nap state as long as A_PWD is low with BUSY
being low and sampling has not started. The minimum acquisition time for the first sampling after the nap state
is 60 nsec more than t(acq) as defined in the timing requirements section.
A_PWD
(Held Low)
BUSY
SAMPLE
(Internal)
t(acq) + 60 ns
NAP
(Internal Active High)
NOTE: The SAMPLE (Internal) signal is generated as described in the Sampling and Conversion
Start section.
Figure 9. Device Operation While A_PWD is Held Low
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SLAS410 − DECEMBER 2003
A_PWD
BUSY
SAMPLE
(Internal)
t(acq) + 60 ns
NAP
(Internal Active High)
NOTE: The SAMPLE (Internal) signal is generated as described in the Sampling and Conversion
Start section.
Figure 10. Device Operation While A_PWD is Toggling
POWERDOWN/RESET
A low level on the PWD/RST pin puts the device in the powerdown phase. This is an asynchronous signal. As
shown in Figure 11, the device is in the reset phase for the first tw6 period after a high-to-low transition of
PWD/RST. During this period the output code is 3F80 (hex) to indicate that the device is in the reset phase. The
device powers down if the PWD/RST pin continues to be low for a period of more than tw7. Data is not valid for
the first four conversions after a power-up (see Figure 11) or an end of reset (see Figure 12). The device is
initialized during the first four conversions.
tw7
Valid Conversions
PWD/RST
First 4 Invalid Conversions
BUSY
1
2
3
4
5
td13
D13−D0
11 1111 1000 0000
Power Down
Phase
RESET Phase
Invalid Data
Valid Data
Figure 11. Device Power Down
tw6
45 ns
PWD/RST
Valid Conversions
First 4 Invalid Conversions
BUSY
D13−D0
1
3
4
5
11 1111 1000 0000
RESET Phase
Figure 12. Device Reset
12
2
Invalid Data
Valid Data
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SLAS410 − DECEMBER 2003
TYPICAL CHARACTERISTICS(1)
EFFECTIVE NUMBER OF BITS
vs
FREE-AIR TEMPERATURE
DC CODE SPREAD AT CENTER OF CODE
13
2664
2500
+VA = 5 V,
+VBD = 5 V,
Code = 8193,
Sample Rate = 3 MSPS,
Vref = 2.5 V,
TA = 255C
Count
2000
1500
1350
1000
758
500
208
0
0
18
2
ENOB − Effective Number of Bits − Bits
3000
0
12.9
12.8
12.7
12.6
12.5
12.4
12.3
12.2
12.1
12
−40
8190 8191 8192 8193 8194 8195 8196 8197
Code
Figure 13
−20
0
20
40
60
TA − Free-Air Temperature − °C
80
Figure 14
SIGNAL-TO-NOISE AND DISTORTION
vs
FREE-AIR TEMPERATURE
SIGNAL-TO-NOISE RATIO
vs
FREE-AIR TEMPERATURE
79
79
78.5
SNR − Signal-to-Noise Ratio − dB
SINAD − Signal-to-Noise and Distortion − dB
fi = 100 kHz,
Sample Rate = 3 MSPS,
+VA = 5 V,
+VBD = 5 V,
Vref = 2.5 V
78
77
fi = 100 kHz,
Sample Rate = 3 MSPS,
+VA = 5 V,
+VBD = 5 V,
Vref = 2.5 V
76
75
−40
−20
0
20
40
60
TA − Free-Air Temperature − °C
Figure 15
78
77.5
77
76.5
76
75.5
80
75
−40
fi = 100 kHz,
Sample Rate = 3 MSPS,
+VA = 5 V,
+VBD = 5 V,
Vref = 2.5 V
−20
0
20
40
60
TA − Free-Air Temperature − °C
80
Figure 16
(1) At Vref = 2.5 V external, unless otherwise specified.
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SLAS410 − DECEMBER 2003
TOTAL HARMONIC DISTORTION
vs
FREE-AIR TEMPERATURE
SPURIOUS FREE DYNAMIC RANGE
vs
FREE-AIR TEMPERATURE
−80
THD − Total Harmonic Distortion − dB
SFDR − Spurious Free Dynamic Range − dB
105
100
95
90
fi = 100 kHz,
Sample Rate = 3 MSPS,
+VA = 5 V,
+VBD = 5 V,
Vref = 2.5 V
85
80
−40
−20
0
20
40
60
TA − Free-Air Temperature − °C
fi = 100 kHz,
Sample Rate = 3 MSPS
+VA = 5 V,
+VBD = 5 V,
Vref = 2.5 V
−85
−90
−95
−100
−105
−40
80
−20
0
20
40
60
TA − Free-Air Temperature − °C
Figure 17
Figure 18
SIGNAL-TO-NOISE AND DISTORTION
vs
INPUT FREQUENCY
EFFECTIVE NUMBER OF BITS
vs
INPUT FREQUENCY
79
TA = 255C,
Sample Rate = 3 MSPS,
+VA = 5 V,
+VBD = 5 V,
Vref = 2.5 V
13.5
13
SINAD − Signal-to-Noise and Distortion − dB
ENOB − Effective Number of Bits − Bits
14
12.5
12
11.5
11
10.5
78
77
76
75
74
TA = 255C,
Sample Rate = 3 MSPS,
+VA = 5 V,
+VBD = 5 V,
Vref = 2.5 V
73
72
10
0
200
400
600 800 1000 1200
fi − Input Frequency − kHz
Figure 19
14
80
1400
0
200
400
600
800 1000
fi − Input Frequency − kHz
Figure 20
1200
1400
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SLAS410 − DECEMBER 2003
SIGNAL-TO-NOISE RATIO
vs
INPUT FREQUENCY
SPURIOUS FREE DYNAMIC RANGE
vs
INPUT FREQUENCY
105
SNR − Signal-to-Noise Ratio − dB
78
77
76
75
TA = 255C,
Sample Rate = 3 MSPS,
+VA = 5 V,
+VBD = 5 V,
Vref = 2.5 V
74
73
SFDR − Spurious Free Dynamic Range − dB
79
TA = 255C,
Sample Rate = 3 MSPS,
+VA = 5 V,
+VBD = 5 V,
Vref = 2.5 V
100
95
90
85
80
75
70
72
0
200
0
400
600 800 1000 1200 1400
fi − Input Frequency − kHz
200
400
Figure 21
800
1000 1200 1400
Figure 22
TOTAL HARMONIC DISTORTION
vs
INPUT FREQUENCY
GAIN ERROR
vs
SUPPLY VOLTAGE
−70
1
TA = 255C,
Sample Rate = 3 MSPS,
+VA = 5 V,
+VBD = 5 V,
Vref = 2.5 V
−75
−80
−85
−90
0.8
0.6
E G − Gain Error − mV
THD − Total Harmonic Distortion − dB
600
fi − Input Frequency − kHz
TA = 255C,
Sample Rate = 3 MSPS,
+VBD = 5 V,
Vref = 2.5 V
0.4
0.2
0
−0.2
−0.4
−95
−0.6
−100
−0.8
−105
0
200
400 600 800 1000 1200 1400
fi − Input Frequency − kHz
Figure 23
−1
4.75
4.85
4.95
5.05
5.15
+VA − Supply Voltage − V
5.25
Figure 24
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SLAS410 − DECEMBER 2003
GAIN ERROR
vs
FREE-AIR TEMPERATURE
OFFSET ERROR
vs
SUPPLY VOLTAGE
1
0.5
TA = 255C,
Sample Rate = 3 MSPS,
+VBD = 5 V,
Vref = 2.5 V
Sample Rate = 3 MSPS,
+VA = 5 V,
+VBD = 5 V,
Vref = 2.5 V
0.75
0.5
E G − Gain Error − mV
E O − Offset Error − mV
1
0
0.25
0
−0.25
−0.5
−0.5
−0.75
−1
4.75
4.85
4.95
5.05
5.15
+VA − Supply Voltage − V
−1
−40
5.25
−20
0
20
40
60
TA − Free-Air Temperature − °C
Figure 25
Figure 26
POWER DISSIPATION
vs
SAMPLE RATE
OFFSET ERROR
vs
FREE-AIR TEMPERATURE
1
0.5
90
Sample Rate = 3 MSPS,
+VA = 5 V,
+VBD = 5 V,
Vref = 2.5 V
0.25
0
−0.25
TA = 255C,
+VA = 5 V,
+VBD = 5 V,
Vref = 2.5 V
80
70
NAP Disabled
60
50
NAP Enabled
40
30
−0.5
20
−0.75
10
−1
−40
0
−20
0
20
40
60
TA − Free-Air Temperature − °C
Figure 27
16
PD − Power Dissipation − mW
E O − Offset Error − mV
0.75
80
80
0
500
1000
1500
2000
Sample Rate − KSPS
Figure 28
2500
3000
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SLAS410 − DECEMBER 2003
DIFFERENTIAL NONLINEARITY
vs
FREE-AIR TEMPERATURE
POWER DISSIPATION
vs
FREE-AIR TEMPERATURE
1
90
88
Sample Rate = 3 MSPS,
+VA = 5 V,
+VBD = 5 V,
Vref = 2.5 V
DNL − Differential Nonlinearity − LSBs
PD − Power Dissipation − mW
89
87
86
85
84
83
82
81
80
−40
−20
0
20
40
60
TA − Free-Air Temperature − °C
0.75
Max
0.5
0.25
Sample Rate = 3 MSPS,
+VA = 5 V,
+VBD = 5 V,
Vref = 2.5 V
0
−0.25
−0.5
Min
−0.75
−1
−40
80
−20
0
Figure 29
2.5045
Max
Vref − Internal Reference Output − V
INL − Integral Nonlinearity − LSBs
80
2.505
0.75
0.5
Sample Rate = 3 MSPS,
+VA = 5 V,
+VBD = 5 V,
Vref = 2.5 V
−0.25
−0.5
Min
−0.75
−1
−40
60
INTERNAL REFERENCE OUTPUT
vs
FREE-AIR TEMPERATURE
1
0
40
Figure 30
INTEGRAL NONLINEARITY
vs
FREE-AIR TEMPERATURE
0.25
20
TA − Free-Air Temperature − °C
+VA = 5 V,
+VBD = 5 V
2.504
2.5035
2.503
2.5025
2.502
2.5015
2.501
2.5005
−20
0
20
40
60
TA − Free-Air Temperature − °C
Figure 31
80
2.5
−40
−20
0
20
40
60
TA − Free-Air Temperature − °C
80
Figure 32
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SLAS410 − DECEMBER 2003
INTERNAL REFERENCE OUTPUT
vs
SUPPLY VOLTAGE
2.505
Vref − Internal Reference Output − V
2.5045
TA = 255C,
+VBD = 5 V
2.504
2.5035
2.503
2.5025
2.502
2.5015
2.501
2.5005
2.5
4.75
4.85
4.95
5.05
5.15
5.25
+VA − Supply Voltage − V
Figure 33
DIFFERENTIAL NONLINEARITY
DNL − LSB
1
0.8
TA = 255C, +VA = 5 V, +VBD = 5 V,
Sample Rate = 3 MSPS, Vref = 2.5 V
0.6
0.4
0.2
0
−0.2
−0.4
−0.6
−0.8
−1
0
2000
4000
6000
8000
Code
10000
12000
14000
16000
Figure 34
INTEGRAL NONLINEARITY
1.5
TA = 255C, +VA = 5 V, +VBD = 5 V,
Sample Rate = 3 MSPS, Vref = 2.5 V
INL − LSB
1
0.5
0
−0.5
−1
−1.5
0
2000
4000
6000
8000
Code
Figure 35
18
10000
12000
14000
16000
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SLAS410 − DECEMBER 2003
FFT
Signal Power − dB
20
TA = 255C, +VA = 5 V, +VBD = 5 V,
Sample Rate = 3 MSPS, Vref = 2.5 V,
fi = 0.99 MHz
0
−20
−40
−60
−80
−100
−120
−140
−160
−180
0
0.2
0.4
0.6
0.8
1
1.2
1.4
fi − Input Frequency − MHz
Figure 36
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SLAS410 − DECEMBER 2003
PRINCIPLES OF OPERATION
The ADS7891 is a member of a family of high-speed successive approximation register (SAR) analog-to-digital
converters (ADC). The architecture is based on charge redistribution, which inherently includes a sample/hold
function.
The conversion clock is generated internally. The conversion time is 273 ns max (at 5 V +VBD).
The analog input is provided to two input pins: +IN and −IN. (Note that this is pseudo differential input and there
are restrictions on –IN voltage range.) When a conversion is initiated, the difference voltage between these pins
is sampled on the internal capacitor array. While a conversion is in progress, both inputs are disconnected from
any internal function.
REFERENCE
The ADS7891 has a built-in 2.5-V (nominal value) reference but can operate with an external reference. When
an internal reference is used, pin 2 (REFOUT) should be connected to pin 1 (REFIN) with an 0.1-µF decoupling
capacitor and a 1-µF storage capacitor between pin 2 (REFOUT) and pins 47, 48 (REFM). The internal
reference of the converter is buffered . There is also a buffer from REFIN to CDAC. This buffer provides isolation
between the external reference and the CDAC and also recharges the CDAC during conversion. It is essential
to decouple REFOUT to AGND with a 0.1-µF capacitor while the device operates with an external reference.
ANALOG INPUT
When the converter enters hold mode, the voltage difference between the +IN and −IN inputs is captured on
the internal capacitor array. The voltage on the −IN input is limited to between –0.2 V and 0.2 V, thus allowing
the input to reject a small signal which is common to both the +IN and −IN inputs. The +IN input has a range
of –0.2 V to (+Vref +0.2 V). The input span (+IN – (−IN)) is limited from 0 V to VREF.
The input current on the analog inputs depends upon a number of factors: sample rate, input voltage, signal
frequency, and source impedance. Essentially, the current into the ADS7891 charges the internal capacitor
array during the sample period. After this capacitance has been fully charged, there is no further input current
(this may not happen when a signal is moving continuously). The source of the analog input voltage must be
able to charge the input capacitance (27 pF) to better than a 14-bit settling level with a step input within the
acquisition time of the device. The step size can be selected equal to the maximum voltage difference between
two consecutive samples at the maximum signal frequency. (Refer to Figure 39 for the suggested input circuit.)
When the converter goes into hold mode, the input impedance is greater than 1 GΩ.
Care must be taken regarding the absolute analog input voltage. To maintain the linearity of the converter, both
−IN and +IN inputs should be within the limits specified. Outside of these ranges, the converter’s linearity may
not meet specifications.
Care should be taken to ensure that +IN and −IN see the same impedance to the respective sources. (For
example, both +IN and −IN are connected to a decoupling capacitor through a 21-Ω resistor as shown in
Figure 39.) If this is not observed, the two inputs could have different settling times. This may result in an offset
error, gain error, or linearity error which changes with temperature and input voltage.
RECOMMENDED OPERATIONAL AMPLIFIERS
It is recommended to use the THS4031 or THS4211 op amps for the analog input. All of the performance figures
in this data sheet are measured using the THS4031. Refer to Figure 39 for more information.
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SLAS410 − DECEMBER 2003
DIGITAL INTERFACE
TIMING AND CONTROL
Refer to the SAMPLING AND CONVERSION START section and the CONVERSION ABORT section.
READING DATA
The ADS7891 outputs full parallel data in straight binary format as shown in Table 1. The parallel output is active
when CS and RD are both low. There is a minimal quiet sampling period requirement around the falling edge
of CONVST as stated in the timing requirements section. Data reads or bus three-state operations should not
be attempted within this period. Any other combination of CS and RD three-states the parallel output. Refer to
Table 1 for ideal output codes.
Table 1. Ideal Input Voltages and Output Codes(1)
DESCRIPTION
Full scale
Midscale
Midscale − 1 LSB
ANALOG VALUE
BINARY CODE
HEX CODE
Vref − 1 LSB
Vref/2
11 1111 1111 1111
3FFF
10 0000 0000 0000
2000
Vref/2 − 1 LSB
0V
01 1111 1111 1111
1FFF
Zero
00 0000 0000 0000
0000
(1) Full-scale range = Vref and least significant bit (LSB) = Vref/16384
The output data appears as a full 14-bit word (D13−D0) on pins DB13 – DB0 (MSB−LSB) if BYTE is low.
READING THE DATA IN BYTE MODE
The result can also be read on an 8-bit bus for convenience by using pins DB13−DB6. In this case two reads
are necessary; the first as before, leaving BYTE low and reading the 8 most significant bits on pins DB13−DB6,
and then bringing BYTE high. When BYTE is high, the lower bits (D5−D0) followed by all zeros are on pins DB13
− DB6 (refer to Table 2).
These multi-word read operations can be performed with multiple active RD signals (toggling) or with RD tied
low for simplicity.
Table 2. Conversion Data Read Out
DATA READ OUT
BYTE
DB13 − DB6
DB5 − DB0
High
D5 − D0, 00
All zeroes
Low
D13 − D6
D5 − D0
Also refer to the DATA READ and DEVICE OPERATION AND DATA READ IN BACK-TO-BACK
CONVERSION sections for more details.
Reset
Refer to the POWERDOWN/RESET section for the device reset sequence.
It is recommended to reset the device after power on. A reset can be issued once the power has reached 95%
of its final value.
PWD/RST is an asynchronous active low input signal. A current conversion is aborted no later than 45 ns after
the converter is in the reset mode. In addition, the device outputs a 3F80 code to indicate a reset condition. The
converter returns back to normal operation mode immediately after the PWD/RST input is brought high.
Data is not valid for the first four conversions after a device reset.
Powerdown
Refer to the POWERDOWN/RESET section for the device powerdown sequence.
The device enters powerdown mode if a PWD/RST low duration is extended for more than a period of tw7.
The converter goes back to normal operation mode no later than a period of td13 after the PWD/RST input is
brought high.
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SLAS410 − DECEMBER 2003
After this period, normal conversion and sampling operation can be started as discussed in previous sections.
Data is not valid for the first four conversions after a device reset.
Nap Mode
Refer to the NAP MODE section in the DESCRIPTION AND TIMING DIAGRAMS section for information.
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SLAS410 − DECEMBER 2003
APPLICATION INFORMATION
LAYOUT
For optimum performance, care should be taken with the physical layout of the ADS7891 circuitry.
As the ADS7891 offers single-supply operation, it is often used in close proximity with digital logic,
micro-controllers, microprocessors, and digital signal processors. The more digital logic present in the design
and the higher the switching speed, the more difficult it is to achieve acceptable performance from the converter.
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 the end of sampling (within quiet sampling time) and just
prior to latching the output of the analog comparator during the conversion phase. Thus, driving any single
conversion for an n-bit SAR converter, there are n+1 windows in which large external transient voltages can
affect the conversion result. Such glitches might originate from switching power supplies, nearby digital logic,
or 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.
On average, the ADS7891 draws very little current from an external reference as the reference voltage is
internally buffered. 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. A 0.1-µF bypass capacitor and 1-µF storage capacitor
are recommended from REFIN (pin 1) directly to REFM (pin 48).
The AGND and BDGND pins should be connected to a clean ground point. In all cases, this should be the analog
ground. Avoid connections which are too close to the grounding point of a micro-controller or digital signal
processor. If required, run a ground trace directly from the converter to the power supply entry point. The ideal
layout consists of an analog ground plane dedicated to the converter and associated analog circuitry.
As with the AGND connections, +VA should be connected to a 5-V power supply plane that is separate from
the connection for +VBD and digital logic until they are connected at the power entry point onto the PCB. Power
to the ADS7891 should be clean and well bypassed. A 0.1-µF ceramic bypass capacitor should be placed as
close to the device as possible. See Table 3 for the placement of capacitor. In addition to a 0.1-µF capacitor,
a 1-µF capacitor is recommended. In some situations, additional bypassing may be required, such as a 100-µF
electrolytic capacitor or even a Pi filter made up of inductors and capacitors, all designed to essentially low-pass
filter the 5-V supply, removing the high frequency noise.
Table 3. Power Supply Decoupling Capacitor Placement
POWER SUPPLY PLANE
CONVERTER ANALOG SIDE
SUPPLY PINS
Pairs of pins that require a shortest path to decoupling
capacitors
(4,5), (9,8), (10,11), (13, 15), (43, 44) (46, 45)
Pins that require no decoupling
14, 12
Analog 5 V
CONVERTER DIGITAL SIDE
(24, 25), (34, 35)
+VA
0.1 µF
1 µF
ADS7891
AGND
AGND
0.1 µF
REFOUT
External
Reference in
REFIN
1 µF
0.1 µF
REFM
AGND
21 Ω
Analog Input
Circuit
21 Ω
+IN
−IN
Figure 37. Using External Reference
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SLAS410 − DECEMBER 2003
Analog 5 V
+VA
0.1 µF
1 µF
ADS7891
AGND
AGND
REFOUT
0.1 µF
1 µF
REFIN
REFM
AGND
21 Ω
Analog Input
Circuit
21 Ω
+IN
−IN
Figure 38. Using Internal Reference
130 pF
1 kΩ
Bipolar Signal Input (+1.25 Vp−p)
2.5 V DC
1 kΩ
_
100 Ω
3 kΩ
12 Ω
21 Ω
THS4031
+
1 kΩ
21 Ω
680 pF
1 nF
AGND
AGND
Figure 39. Typical Analog Input Circuit
GPIO
CS
GPIO
BYTE
GPIO
CONVST
Microcontroller
ADS7891
P[7:0]
DB[13:6]
RD
RD
INT
BUSY
Figure 40. Interfacing With Microcontroller
24
+IN
ADS7891
−IN
MECHANICAL DATA
MTQF019A – JANUARY 1995 – REVISED JANUARY 1998
PFB (S-PQFP-G48)
PLASTIC QUAD FLATPACK
0,27
0,17
0,50
36
0,08 M
25
37
24
48
13
0,13 NOM
1
12
5,50 TYP
7,20
SQ
6,80
9,20
SQ
8,80
Gage Plane
0,25
0,05 MIN
0°– 7°
1,05
0,95
Seating Plane
0,75
0,45
0,08
1,20 MAX
4073176 / B 10/96
NOTES: A. All linear dimensions are in millimeters.
B. This drawing is subject to change without notice.
C. Falls within JEDEC MS-026
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