®
®
CDS-1402
14-Bit, Very Fast-Settling Correlated Double Sampling Circuit
PRODUCT OVERVIEW
The CDS-1402 is an application-specific, cor-
related double sampling (CDS) circuit designed for
electronic-imaging applications that employ CCD's
(charge coupled devices) as their photodetec-
tor. The CDS-1402 has been optimized for use in
digital video applications that employ 10 to 14-bit
A/D converters. The low-noise CDS-1402 can
accurately determine each pixel's true video signal
level by sequentially sampling the pixel's offset
signal and its video signal and subtracting the two.
The result is that the consequences of residual
charge, charge injection and low-frequency "kTC"
noise on the CCD's output floating capacitor are
effectively eliminated. The CDS-1402 can also be
used as a dual sample-hold amplifier in a data
acquisition system.
The CDS-1402 contains two sample-hold
amplifiers and appropriate support/control circuitry.
Features include independent offset-adjust capabil-
ity for each S/H, adjustment for matching gain
between the two S/H's, and four control lines for
triggering the A/D converter used in conjunction
with the CDS-1402. The CDS circuit's "pingpong"
timing approach (the offset signal of the "n+1"
pixel can be acquired while the video output of the
"nth" pixel is being converted) guarantees a mini-
mum throughput, in a 14-bit application, of 5MHz.
In other words, the true video signal (minus offset)
will be available at the output of the CDS-1402
every 200ns. This correlates with the fact that
an acquisition time of 100ns is required for each
internal S/H amplifier (5V step acquired to ±0.01%
accuracy). The input and output of the CDS-1402
can swing up to ±2.5 Volts.
The functionally complete CDS-1402 is pack-
aged in a single, 24-pin, ceramic DDIP. It operates
from ±5V analog and +5V digital supplies and
typically consumes 350mW. Though the CDS-
1402's approach to CDS appears straightforward
(see Funtional Description ), the circuit actually
exploits an elegant architecture whose tradeoffs
enable it to offer wide-bandwidth, low-noise and
high-throughput combinations unachievable until
now. The CDS-1402, a generic type of circuit, can
be used with most 10 to 14-bit A/D converters.
However, DATEL offers A/D converters optimized for
use with CDS-1402.
FEATURES
with 10 to 14-bit A/D converters
Use
Megapixels/second minimum throughput (14
5
bits)
±2.5V input/output ranges, Gain = –1
noise, 200μVrms
Low
independent S/H amplifiers
Two
matching between S/H's
Gain
Offset adjustments for each S/H
external A/D control lines
Four
Small package, 24-pin ceramic DDIP
power, 350mW
Low
cost
Low
BLOCK DIAGRAM
100k
OFFSET ADJUST V1
DO NOT CONNECT
1
2
500
ANALOG INPUT 1
3
–
500
50
C
H
7
S/H1 ROUT
S/H 1
+
6
S/H1 OUT
OPTIONAL
OFFSET ADJUST V2 9
100k
450
500
DO NOT CONNECT 10
500
ANALOG INPUT 2
4
–
8
S/H2
SUMMING NODE
C
H
S/H 2
+
22 V OUT
S/H1 COMMAND 11
18 A/D CLOCK 1
17 A/D CLOCK 1
S/H2 COMMAND 12
19 A/D CLOCK 2
20 A/D CLOCK 2
5, 14, 21, 23
ANALOG GROUND
24
+5V ANALOG
SUPPLY
13
–5V ANALOG
SUPPLY
16
+5V DIGITAL
SUPPLY
15
DIGITAL
GROUND
Figure 1. CDS-1402 Functional Block Diagram
DATEL
•
11 Cabot Boulevard, Mansfield, MA 02048-1151 USA •
Tel: (508) 339-3000
•
www.datel.com
•
e-mail: help@datel.com
16 Jan 2013
MDA_CDS-1402.B03
Page 1 of 9
®
®
CDS-1402
14-Bit, Very Fast-Settling Correlated Double Sampling Circuit
UNITS
Volts
Volts
Volts
Volts
Volts
°C
PARAMETERS
Operating Temp. Range, Case
CDS-1402MC
CDS-1402MM
Thermal Impedance
jc
ca
Storage Temperature Range
Package Type
Weight
PHYSICAL/ENVIRONMENTAL
MIN.
TYP.
0
–55
—
—
MAX.
+70
+125
UNITS
°C
°C
ABSOLUTE MAXIMUM RATINGS
PARAMETERS
LIMITS
+5V Analog Supply (Pin 24)
0 to +6.3
–5V Analog Supply (Pin 13)
0 to –6.3
+5V Digital Supply (Pin 16)
–0.3 to +6
Digital Inputs (Pins 11, 12)
–0.3 to +VDD +0.3
Analog Inputs (Pins 3, 4)
±3.2
Lead Temperature (10 seconds)
+300
—
5
—
°C/Watt
—
22
—
°C/Watt
–65
—
+150
°C
24-pin, metal-sealed ceramic DDIP
0.42 ounces (12 grams)
FUNCTIONAL SPECIFICATIONS
(T
A
= +25°C, ±V
CC
= ±5V, +V
DD
= +5V, pixel rate = 5MHz, and a minimum warmup time of 2 minutes unless otherwise noted.)
ANALOG INPUTS
➀
Input Voltage Range
Input Resistance
Input Capacitance
DIGITAL INPUT
Logic Levels
Logic "1"
Logic "0"
Logic Loading "1"
Logic Loading "0"
PERFORMANCE
Sample Mode Offset Error - S/H1
Gain Error - S/H1
Pedestal - S/H1
Sample Mode Offset Error - S/H2
Gain Error - S/H2
Pedestal - S/H2
Sample Mode Offset Error - CDS
Differential Gain Error - CDS
Pedestal - CDS
Pixel Rate (14-bit settling)
➁
Input Bandwidth, ±2.5V
Small Signal (–20dB input)
Large Signal (–0.5dB input)
Slew Rate
Aperture Delay Time
Aperture Uncertainty
S/H Acquisition Time
➀
(to ±0.01%, 5V step)
Hold Mode Settling Time
(to ±0.15mV)
Noise
Feedthrough Rejection
Overvoltage Recovery Time
S/H Saturation Voltage
Droop Rate
ANALOG OUTPUTS
➂
Output Voltage Range
Output Impedance
Output Current
DIGITAL OUTPUTS
Logic Levels
Logic "1"
Logic "0"
Logic Loading "1"
Logic Loading "0"
MIN.
±2.5
—
—
+25°C
TYP.
—
500
7
MAX.
—
—
15
MIN.
±2.5
—
—
0 TO +70°C
TYP.
—
500
7
MAX.
—
—
15
MIN.
±2.5
—
—
–55 TO +125°C
TYP.
—
500
7
MAX.
—
—
15
UNITS
Volts
Ohms
pF
+2.0
—
—
—
—
—
—
—
—
—
—
—
—
5
—
—
—
—
—
—
—
—
—
—
—
—
±2.5
—
—
—
—
—
—
±3
±0.5
±5
±3
±0.5
±5
±3
±0.5
±10
—
24
8
±500
±10
±5
90
20
200
72
200
±3.2
±30
—
0.5
—
—
+0.8
+10
–10
±15
±1
±25
±15
±1
±25
±15
±1.5
±25
—
—
—
—
—
—
130
—
—
—
—
—
±50
—
—
±20
+2.0
—
—
—
—
—
—
—
—
—
—
—
—
5
—
—
—
—
—
—
—
—
—
—
—
—
±2.5
—
—
—
—
—
—
±4
±0.7
±10
±4
±0.7
±10
±4
±0.5
±10
—
24
8
±500
±10
±5
90
20
200
72
200
±3.2
±30
—
0.5
—
—
+0.8
+10
–10
±15
±1
±25
±15
±1
±25
±15
±1.5
±25
—
—
—
—
—
—
130
—
—
—
—
—
±100
—
—
±20
+2.0
—
—
—
—
—
—
—
—
—
—
—
—
5
—
—
—
—
—
—
—
—
—
—
—
—
±2.5
—
—
—
—
—
—
±5
±0.75
±15
±5
±0.75
±15
±5
±0.75
±15
—
24
8
±500
±10
±5
90
20
200
72
200
±3.2
±30
—
0.5
—
—
+0.8
+10
–10
±15
±1
±25
±15
±1
±25
±15
±1.5
±30
—
—
—
—
—
—
130
—
—
—
—
—
±100
—
—
±20
Volts
Volts
μA
μA
mV
%
mV
mV
%
mV
mV
%
mV
MSPS
MHz
MHz
V/μs
ns
ps rms
ns
ns
μVrms
dB
ns
V
mV/μs
Volts
Ohms
mA
+3.9
—
—
—
—
—
—
—
—
+0.4
–4
+4
+3.9
—
—
—
—
—
—
—
—
+0.4
–4
+4
+3.9
—
—
—
—
—
—
—
—
+0.4
–4
+4
Volts
Volts
mA
mA
DATEL
•
11 Cabot Boulevard, Mansfield, MA 02048-1151 USA •
Tel: (508) 339-3000
•
www.datel.com
•
e-mail: help@datel.com
16 Jan 2013
MDA_CDS-1402.B03
Page 2 of 9
®
®
CDS-1402
14-Bit, Very Fast-Settling Correlated Double Sampling Circuit
+25°C
TYP.
+5.0
–5.0
+5.0
+21
–16
+2
350
60
MAX.
+5.25
–5.25
+5.25
+50
–50
+5
500
—
MIN.
+4.75
–4.75
+4.75
—
—
—
—
—
0 TO +70°C
TYP.
+5.0
–5.0
+5.0
+21
–16
+2
350
60
MAX.
+5.25
–5.25
+5.25
+50
–50
+5
500
—
MIN.
+4.75
–4.75
+4.75
—
—
—
—
—
–55 TO +125°C
TYP.
+5.0
–5.0
+5.0
+21
–16
+2
350
60
MAX.
+5.25
–5.25
+5.25
+50
–50
+5
500
—
UNITS
Volts
Volts
Volts
mA
mA
mA
mW
dB
POWER REQUIREMENTS
Power Supply Ranges
+5V Analog Supply
–5V Analog Supply
+5V Digital Supply
Power Supply Currents
+5V Analog Supply
–5V Analog Supply
+5V Digital Supply
Power Dissipation
Power Supply Rejection
MIN.
+4.75
–4.75
+4.75
—
—
—
—
—
Footnotes:
➀
Pins 3 and 4.
➁
See Figure 4 for relationship between input voltage, accuracy, and acquisition time.
➂
Pins 6 and 22.
PIN
1
2
3
4
5
6
7
8
9
10
11
12
FUNCTION
INPUT/OUTPUT CONNECTIONS
PIN FUNCTION
24
23
22
21
20
19
18
17
16
15
14
13
+5V ANALOG SUPPLY
ANALOG GROUND
V OUT
ANALOG GROUND
A/D CLOCK2
A/D CLOCK2
A/D CLOCK1
A/D CLOCK1
+5V DIGITAL SUPPLY
DIGITAL GROUND
ANALOG GROUND
–5V ANALOG SUPPLY
OFFSET ADJUST V1
DO NOT CONNECT
ANALOG INPUT 1
ANALOG INPUT 2
ANALOG GROUND
S/H1 OUT
S/H1 ROUT
S/H2 SUMMING NODE
OFFSET ADJUST V2
DO NOT CONNECT
S/H1 COMMAND
S/H2 COMMAND
TECHNICAL NOTES
1. 1. To achieve specified performance, all power supply pins should be
bypassed with 2.2μF tantalum capacitors in parallel with 0.1μF ceramic ca-
pacitors. All ANALOG GROUND (pins 5, 14, 21 and 23) and DIGITAL GROUND
(pin 15) pins should be tied to a large analog ground plane beneath the
package.
2. In the CDS configuration, to avoid saturation of the S/H amplifiers, the
maximum analog inputs and conditions are as follows:
ANALOG INPUT 1 < ±3.2V
(ANALOG INPUT 1 – ANALOG INPUT 2) < ±3.2V
3. The combined video and reference/offset signal from the CCD array must
be applied to S/H2, while the reference/offset signal is applied to S/H1.
4. To use as a CDS circuit, tie pin 8 (S/H2 SUMMING NODE) to either pin 6
(S/H1 OUT), through a 100 Ohm potentiometer, or directly to pin 7 (S/H1
ROUT). In both cases, the CCD's output is tied to pins 3 (ANALOG INPUT 1)
and 4 (ANALOG INPUT 2). As shown in Figure 5, the 100Ω potentiometer is
for gain matching.
5. To use as a dual S/H, leave pin 7 (S/H1 ROUT) and pin 8 (S/H2 SUMMING
NODE) floating. Pin 6 (S/H1 OUT) will be the output of S/H1 and pin 22 (V
OUT) will be the output of S/H2.
6. See Figure 4 for acquisition time versus accuracy and input voltage step
amplitude.
DATEL
•
11 Cabot Boulevard, Mansfield, MA 02048-1151 USA •
Tel: (508) 339-3000
•
www.datel.com
•
e-mail: help@datel.com
16 Jan 2013
MDA_CDS-1402.B03
Page 3 of 9
®
®
CDS-1402
14-Bit, Very Fast-Settling Correlated Double Sampling Circuit
FUNCTIONAL DESCRIPTION
CORRELATED DOUBLE SAMPLING
All photodetector elements (photodiodes, photomultiplier tubes, focal
plane arrays, charge coupled devices, etc.) have unique output character-
istics that call for specific analog-signalprocessing (ASP) functions at their
outputs. Charge coupled devices (CCD’s), in particular, display a number
of unique characteristics. Among them is the fact that the "offset error"
associated with each individual pixel (i.e., the apparent photonic content
of that pixel after having had no light incident upon it) changes each and
every time that particular pixel is accessed.
Most of us think of an offset as a constant parameter that either can
be compensated for (by performing an offset adjustment) or can be
measured, recorded, and subtracted from subsequent readings to yield
more accurate data. Contending with an offset that varies from reading to
reading requires measuring and recording (or capturing and storing) the
offset each and every time, so it can be subtracted from each subsequent
data reading.
The "double sampling" aspect of CDS refers to the operation of sampling
and storing/recording a given pixel’s offset and then sampling the same
pixel’s output an instant later (with both the offset and the video signal
present) and subsequently subtracting the two values to yield what is
referred to as the "valid video" output for that pixel.
The "correlated" in CDS refers to the fact that the two samples must be
taken close together in time because the offset is constantly varying.
Reasons for this phenomena are discussed below.
At the output of all CCD's, transported pixel charge (electrons) is converted
to a voltage by depositing the charge onto a capacitor (usually called
the output or "floating" capacitor). The voltage that develops across this
capacitor is obviously proportional to the amount of deposited charge
(i.e., the number of electrons) according to DV = DQ/C. Once settled, the
resulting capacitor voltage is buffered and brought to the CCD’s output pin
as a signal whose amplitude is proportional to the total number of photons
incident upon the relevant pixel.
After the output signal has been recorded, the floating capacitor is
discharged ("reset", "clamped", "dumped") and made ready to accept
charge from the next pixel. This is when the problems begin. (This is a
somewhat oversimplified explanation in that the floating capacitor is not
usually "discharged" but, in fact, "recharged" to some predetermined dc
voltage, usually called the "reference level". The pixel offset appears as an
output deviation from that reference level.)
The floating capacitor is normally discharged (charged) via a shunt switch
(typically a FET structure) that has a non-zero "on" resistance. When the
switch is on, its effective series resistance exhibits thermal noise (Johnson
noise) due to the random motion of thermally energized charge. Because
the shunt switch is in parallel with the floating capacitor, the instanta-
neous value of the thermal noise (expressed in either Volts or electrons)
appears across the cap. When the shunt switch is opened, charge/voltage
is left on the floating cap.
The magnitude of this "captured noise voltage" is a function of absolute
temperature (T), the value of the floating capacitor (C) and Boltzman’s
constant (k). It is commonly referred to as "kTC" noise.
The second contributor to the constantly varying pixel offsets is the fact
that, at high pixel rates, the floating capacitor never has time to fully
discharge (charge) during the period in which its shunt switch is closed.
There is always some "residual" charge left on the cap, and the amount of
this charge varies as a function of what was the total charge held during
DATEL
•
the previous pixel. This amount of residual charge is, in fact, deterministic
(if you know the previous charge and the number of time constants in the
discharge period), however, it is less of a contributor than "kTC" noise.
The third major contributor to pixel offset is the fact that as the shunt FET
is turned off, the voltage across (and the charge stored on) its parasitic
junction capacitances changes. The result is an "injection" of excess
charge onto the floating cap causing a voltage step normally called a
"pedestal". The fourth major contributor to pixel offset is a low-frequency
noise component (usually called 1/f noise or pink noise) associated with
the CCD's output buffer amplifier.
Due to all of these contributing factors, "pixel offsets" vary from sample to
sample in an inconsistent, unpredictable manner.
TRADITIONAL APPROACH TO CDS
There are a number of techniques for dealing with the varying-offset
idiosyncrasy of CCD's. The most prevalent has been what can be called
the "sample-sample-subtract" technique. This approach requires the use
of two high-speed sample-hold (S/H) amplifiers and a difference amplifier.
The first S/H is used to acquire and hold a given pixel's offset. Imme-
diately after that, the second S/H acquires and holds the same pixel’s
offset+video signal. After both the S/H outputs have fully settled, the
difference amplifier subtracts the offset from the offset+video yielding the
valid video signal.
CDS-1402 APPROACH (SEE FIGURE 1)
The DATEL CDS-1402 takes a slightly different, though clearly superior,
approach to CDS. It can be called the “sample-subtract-sample” approach.
Note that the CDS-1402 has been configured to offer the greatest amount
of user flexibility. Its two S/H circuits function independently. They have
separate input and output pins. Each has its own independent control
lines. The control-line signals are delayed, buffered, and brought back out
of the package so they can be used to control other circuit functions. Each
S/H has two pins for offset adjusting (if required), one for current and one
for voltage.
In normal operation, the output signal of the CCD is applied simultane-
ously to the inputs (pins 3 and 4) of both S/H amplifiers. S/H1 will normally
be used to capture and hold each pixel’s offset signal. Therefore, S/H1
is initially in its signal-acquisition mode (logic "1" applied to pin 11, S/
H1 COMMAND). This is also called the sample or track mode. Following a
brief interval during which the output of the CCD and the output of S/H1
are allowed to settle, S/H1 is driven into its hold mode by applying a logic
"0" to pin 11. S/H1 is now holding the pixel's offset value.
In most straightforward configurations, the output of S/H1 is connected to
the summing node of S/H2 by connecting pin 7 (S/H1 ROUT) to pin 8 (S/H2
SUMMING NODE).
When the offset+video signal appears at the output of the CCD, S/H2 is
driven into its signal acquisition mode by applying a logic "1" to pin 12
(S/H2 COMMAND).
S/H2 employs a current-summing architecture that subtracts the output of
S/H1 (the offset) from the output of the CCD (offset+video) while acquiring
only the difference signal (i.e., the valid video). A logic "0" subsequently
applied to pin 12 drives S/H2 into its hold mode, and after a brief transient
settling time, the valid video signal appears at pin 22 (V OUT).
11 Cabot Boulevard, Mansfield, MA 02048-1151 USA •
Tel: (508) 339-3000
•
www.datel.com
•
e-mail: help@datel.com
16 Jan 2013
MDA_CDS-1402.B03
Page 4 of 9
®
®
CDS-1402
14-Bit, Very Fast-Settling Correlated Double Sampling Circuit
RESET N
RESET N+1
(CCD OUTPUT)
ANALOG INPUT FOR CDS
(Pins 3 and 4 are tied together)
OFFSET N
OFFSET +
VIDEO N
OFFSET N+1
V
S/H1 (Pin 11)
S/H2 (Pin 12)
100ns typ.
HOLD
100ns typ.
30ns typ.
A/D CLOCK 1 (Pin 17)
HOLD
A/D CLOCK 1 (Pin 18)
A/D CLOCK 2 (Pin 19)
30ns typ.
A/D CLOCK 2 (Pin 20)
VOLTAGE OUTPUT (Pin 22)
VIDEO SIGNAL N-1
VIDEO SIGNAL N
Figure 2. CDS-1402 Typical Timing Diagram
TIMING NOTES
See Figure 2, Typical Timing Diagram. It is advisable that neither of the
CDS-1402's S/H amplifiers be in their sample/ track mode when large,
high-speed transients (normally associated with clock edges) are occurring
throughout the system. This could result in the S/H amplifiers being driven
into saturation, and they may not recover in time to accurately acquire their
next signal.
For example, S/H1 should not be commanded into the sample mode until
all transients associated with the opening of the shunt switch have begun
to decay. Similarly, S/H2 should not be driven into the sample mode until
all transients associated with the clocking of pixel charge onto the output
capacitor have begun to decay. Therefore, it is generally not a good practice
to use the same clock edge to drive S/H1 into hold (holding the offset) and
S/H2 into sample (to acquire the offset + video signal).
S/H's that are in their signal-acquisition modes should be left there as long
as possible (so all signals can settle) and be driven into their hold modes
before any system transients occur. In Figure 2, S/H1 is driven into the
sample mode shortly after the transient from the shunt switch has begun
to decay. S/H1 is then kept in the sample mode while the offset signal and
the S/H output settle. S/H1 is driven into hold just prior to the system clock
pulse(s) that transfers the next pixel charge onto the output capacitor.
As soon as the transients/noise associated with the charge transport begins
to decay, S/H2 can be driven into the sample mode. S/H2 can then be left in
the sample mode until just before the reset pulse for the output capacitor.
In Figure 2, S/H's 1 and 2 both have the same acquisition time. If the pixel-
to-pixel amplitude variation of offset signals is much less than that of video
signals, it may not be necessary for the allocated acquisition time of S/H1
to be as long as that of S/H2.
As shown in the plot (Figure 4) of acquisition times vs. input signal step
size, the S/H's internal to the CDS-1402 acquire smaller-amplitude signals
quicker than they acquire largeramplitude signals. In "maximum-through-
put" applications, assuming "asymmetric" timing can be accommodated,
each S/H should only be given the time it requires, and no more, to acquire
its input signal. Leaving a S/H amp in the sample mode for a longer period
of time has little added benefit.
As an example, the graph shows that it takes 32ns to acquire a 500mV step
to within 10mV of accuracy and 73ns to acquire a 500mV step to within
0.5mV of accuracy. The figures in this graph are typical values at room
temperature.
The CDS-1402 brings out 4 control lines that can be used to trigger an A/D
converter connected to its output. If the A/D is a sampling type, system tim-
ing should be such that the A/D's input S/H amplifier is acquiring the output
of the CDS-1402 at the same time the output is settling to its final value.
For most sampling A/D's, the rising edge of the start-convert pulse drives
the internal S/H into the hold mode under the assumption the S/H has al-
ready fully acquired and is tracking the input signal. In this case, the same
edge can not be used to drive S/H2 into the hold mode and simultaneously
initiate the A/D conversion. The output of S/H2 needs time to settle its
sample-to-hold switching transient, and the input S/H of the A/D needs time
to fully acquire its new input signal.
As shown in Figure 1, output line A/D CLOCK1 (pin 18) is a slightly delayed
version of the signal applied to pin 11 (S/H1 COMMAND), and A/D CLOCK1
(pin 17) is its complement. A/D CLOCK2 (pin 19) is a delayed version of the
signal applied to pin 12 (S/H2 COMMAND), and A/D CLOCK2 (pin 20) is its
complement. Any one of these signals, as appropriate, may be used to trig-
ger the A/D conversion.
Figure 3 is a typical timing diagram for a CDS-1402 in front of DATEL's 14-
bit, 5MHz sampling A/D, the ADS-944.
DATEL
•
11 Cabot Boulevard, Mansfield, MA 02048-1151 USA •
Tel: (508) 339-3000
•
www.datel.com
•
e-mail: help@datel.com
16 Jan 2013
MDA_CDS-1402.B03
Page 5 of 9
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