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Part: CDS-1401MC
Category:
Multimedia
-> Video
-> Imaging
-> Image Signal Conditioner
Description: 14-bit, Fast-settling Correlated Double Sampling Circuit
Company: Datel, Inc.
Datasheet: Download CDS-1401MC datasheet File size : 1121 kB
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CDS-1401
14-Bit, Fast-Settling Correlated Double Sampling Circuit
INNOVATION and EXCELLENCE
FEATURES
· · · · · · · · · · · Use with 10 to 14-bit A/D converters 1.25 Megapixels/second minimum throughput (14 bits) ±10V input/output ranges, Gain = 1 Low noise, 200µVrms Two independent S/H amplifiers Gain matching between S/H's Offset adjustments for each S/H Four external A/D control lines Small package, 24-pin ceramic DDIP Low power, 700mW Low cost
INPUT/OUTPUT CONNECTIONS
PIN 1 2 3 4 5 6 7 8 9 10 11 12 FUNCTION OFFSET ADJUST V1 OFFSET ADJUST I1 ANALOG INPUT 1 ANALOG INPUT 2 ANALOG GROUND S/H1 OUT S/H1 ROUT S/H2 SUMMING NODE OFFSET ADJUST V2 OFFSET ADJUST I2 S/H1 COMMAND S/H2 COMMAND PIN 24 23 22 21 20 19 18 17 16 15 14 13 FUNCTION +15V 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 15V ANALOG SUPPLY
GENERAL DESCRIPTION
The CDS-1401 is an application-specific, correlated double sampling (CDS) circuit designed for electronic-imaging applications that employ CCD's (charge coupled devices) as their photodetector. The CDS-1401 has been optimized for use in digital video applications that employ 10 to 14-bit A/D conver ters. The low-noise CDS-1401 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-1401 can also be used as a dual sample-hold amplifier in a data acquisition system. The CDS-1401 contains two sample-hold amplifiers and appropriate support/control circuitry. Features include independent offset-adjust capability 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-1401. The CDS circuit's "ping-pong" 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 minimum throughput, in a 14-bit application, of 1.25MHz. In other words, the true video signal (minus offset) will be available (continued on page 4-5)
100k OFFSET ADJUST V1 1
1k 100
OFFSET ADJUST I1 2 1k ANALOG INPUT 1 3 CH = 100pF S/H 1 +
7
S/H1 ROUT
6
S/H1 OUT
OPTIONAL 100k OFFSET ADJUST V2 9 1k OFFSET ADJUST I2 10 1k ANALOG INPUT 2 4 CH = 100pF S/H 2 + 8 S/H2 SUMMING NODE 900
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 +15V SUPPLY
13 15V SUPPLY
16 +5V DIGITAL SUPPLY
15 DIGITAL GROUND
Figure 1. CDS-1401 Functional Block Diagram
DATEL, Inc., 11 Cabot Boulevard, Mansfield, MA 02048-1151 (U.S.A.) · Tel: (508) 339-3000 Fax: (508) 339-6356 · For immediate assistance (800) 233-2765
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CDS-1401
ABSOLUTE MAXIMUM RATINGS
PARAMETERS +15V Supply (Pin 24) 15V Supply (Pin 13) +5V Supply (Pin 16) Digital Inputs (Pins 11, 12) Analog Inputs (Pins 3, 4) Lead Temp. (10 seconds) LIMITS 0 to +16 0 to 16 0 to +6 0.3 to +VDD +0.3 ±12 300 UNITS Volts Volts Volts Volts Volts °C
PHYSICAL/ENVIRONMENTAL
PARAMETERS Operating Temp. Range, Case CDS-1401MC CDS-1401MM Thermal Impedance jc ca Storage Temperature Range Package Type Weight MIN. 0 55 -- -- 65 TYP. -- -- 5 22 -- MAX. +70 +125 -- -- +150 UNITS °C °C °C/W °C/W °C
24-pin, metal-sealed, ceramic DDIP 0.42 ounces(12 grams)
FUNCTIONAL SPECIFICATIONS
(TA = +25°C, ±Vcc = ±15V, +VDD = +5V, pixel rate = 1.25MHz, and a minimum warmup time of two minutes unless otherwise noted.)
+25°C ANALOG INPUTS Input Voltage Range Input Resistance Input Capacitance DIGITAL INPUTS 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) Input Bandwidth, ±5V Small Signal (20dB input) Large Signal (0.5dB input) Slew Rate Aper ture Delay Time Aper ture Uncertainty S/H Acquisition Time (to ±0.003%, 10V step) Hold Mode Settling Time (to ±0.15mV) Noise Feedthrough Rejection Overvoltage Recovery Time S/H Saturation Voltage Droop Rate ANALOG OUTPUTS Output Voltage Range Output Impedance Output Current DIGITAL OUTPUTS Logic Levels Logic "1" Logic "0" Logic Loading "1" Logic Loading "0" Pins 3 and 4. +3.9 -- -- -- -- -- -- -- -- +0.4 4 +4 +3.9 -- -- -- -- -- -- -- -- +0.4 4 +4 +3.9 -- -- -- -- -- -- -- -- +0.4 4 +4 Volts Volts mA mA ±10 -- -- -- 0.5 -- -- -- ±20 ±10 -- -- -- 0.5 -- -- -- ±20 ±10 -- -- -- 0.5 -- -- -- ±20 Volts Ohms mA -- -- -- -- -- -- -- -- -- 1.25 -- -- -- -- -- -- -- -- -- -- -- -- ±1 ±0.2 ±15 ±1 ±0.2 ±15 ±1 ±0.25 ±15 -- 7 5 ±80 10 5 340 TBD 200 72 400 ±12.5 ±0.004 ±10 ±1 ±35 ±10 ±1 ±35 ±10 ±1 ±35 -- -- -- -- -- -- 400 -- -- -- -- -- ±0.02 -- -- -- -- -- -- -- -- -- 1.25 -- -- -- -- -- -- -- -- -- -- -- -- ±2 ±0.25 ±15 ±2 ±0.25 ±15 ±2 ±0.3 ±15 -- 7 5 ±80 10 5 350 TBD 200 72 400 ±12.5 ±0.4 ±10 ±1 ±35 ±10 ±1 ±35 ±10 ±1 ±35 -- -- -- -- -- -- 400 -- -- -- -- -- ±2 -- -- -- -- -- -- -- -- -- 1.25 -- -- -- -- -- -- -- -- -- -- -- -- ±4 ±0.3 ±15 ±4 ±0.3 ±15 ±4 ±0.35 ±15 -- 7 5 ±80 10 5 350 TBD 200 72 400 ±12.5 ±0.8 ±10 ±1.5 ±35 ±10 ±1.5 ±35 ±10 ±1.5 ±35 -- -- -- -- -- -- 400 -- -- -- -- -- ±4 mV % mV mV % mV mV % mV MHz MHz MHz V/µs ns ps rms ns ns µVr ms dB ns Volts mV/µs +2 -- -- -- -- -- -- -- -- +0.8 +10 10 +2 -- -- -- -- -- -- -- -- +0.8 +10 10 +2 -- -- -- -- -- -- -- -- +0.8 +10 10 Volts Volts µA µA MIN. ±10 -- -- TYP. -- 1000 7 MAX. -- -- 15 MIN. ±10 -- -- 0 to +70°C TYP. -- 1000 7 MAX. -- -- 15 MIN. ±10 -- -- 55 to +125°C TYP. -- 1000 7 MAX. -- -- 15 UNITS Volts Ohms pF
See Figure 4 for relationship between input voltage, accuracy, and acquisition time.
Pins 6 and 22.
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+25°C POWER REQUIREMENTS Power Supply Ranges +15V Supply 15V Supply +5V Supply Power Supply Currents +15V Supply 15V Supply +5V Supply Power Dissipation Power Supply Rejection MIN. +14.75 14.75 +4.75 -- -- -- -- -- TYP. +15.0 15.0 +5.0 +23 23 +1 700 100 MAX. +15.25 15.25 +5.25 +27 27 +2 850 -- MIN. +14.75 14.75 +4.75 -- -- -- -- -- 0 to +70°C TYP. +15.0 15.0 +5.0 +23 23 +1 700 100 MAX. +15.25 15.25 +5.25 +27 27 +2 850 -- MIN. +14.75 14.75 +4.75 -- -- -- -- -- 55 to +125°C TYP. +15.0 15.0 +5.0 +23 23 +1 700 100 MAX. +15.25 15.25 +5.25 +27 27 +2 850 -- UNITS Volts Volts Volts mA mA mA mW dB
GENERAL DESCRIPTION (continued)
at the output of the CDS-1401 every 800ns. This correlates with the fact that an acquisition time of 400ns is required for each internal S/H amplifier (10V step setting to ±0.003%). The input and output of the CDS-1401 can swing up to ±10 Volts. The functionally complete CDS-1401 is packaged in a single, 24-pin, ceramic DDIP. It operates from ±15V and +5V supplies and consumes 700mW. Though the CDS-1401's approach to CDS appears straightforward (see Description of Operation), the circuit actually exploits an elegant architecture whose tradeoffs enable it to offer wide-bandwidth, low-noise and highthroughput combinations unachievable until now. The CDS1401 is a generic type of circuit that can be used with almost any 10 to 14-bit A/D converter. However, DATEL does offer A/D converters that are optimized for use with the CDS-1401.
FUNCTIONAL DESCRIPTION Correlated Double Sampling
All photodetector elements (photodiodes, photomultiplier tubes, focal plane arrays, charge coupled devices, etc.) have unique output characteristics 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 V = Q/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
TECHNICAL NOTES
1. To achieve specified performance, all power supply pins should be bypassed with 2.2µF tantalum capacitors in parallel with 0.1µF ceramic capacitors. 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 < ±12V (ANALOG INPUT 1 ANALOG INPUT 2) < ±12V 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 200 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 200 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.
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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 instantaneous 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 the previous pixel. This amount of residual charge is, in fact, deter ministic (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 varyingoffset 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. Immediately 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-1401 Approach (See Figure 1) The DATEL CDS-1401 takes a slightly different, though clearly superior, approach to CDS. It can be called the "samplesubtract-sample" approach. Note that the CDS-1401 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
RESET N
RESET N+1
(CCD OUTPUT) ANALOG INPUT FOR CDS (Pins 3 and 4 are tied) S/H 1 (Pin 11) S/H 2 (Pin 12)
OFFSET N OFFSET + VIDEO N
OFFSET N+1
OFFSET + VIDEO N+1
100ns typ.
HOLD
100ns typ. 30ns typ. A/D CLOCK 1 (Pin 17)
HOLD
A/D CLOCK 1 (Pin 18) 30ns typ. A/D CLOCK 2 (Pin 19)
A/D CLOCK 2 (Pin 20)
VIDEO SIGNAL N-1 VOLTAGE OUTPUT
VIDEO SIGNAL N
NOTE: Not Drawn to Scale
Figure 2. CDS-1401 Typical Timing Diagram
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CDS-1401
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 simultaneously 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). Timing Notes See Figure 2, Typical Timing Diagram. It is advisable that neither of the CDS-1401'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 transpor t 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
RESET
RESET
(CCD OUTPUT) ANALOG INPUT FOR CDS (Pins 3 and 4 are tied)
OFFSET (N+1)
OFFSET + VIDEO (N+1)
OFFSET (N+2)
OFFSET + VIDEO (N+2)
S/H1
400ns
S/H2
400ns
30ns typ.
START CONVERT 90ns typ. 420ns EOC OUTPUT DATA
DATA N-1 VALID 73ns max. 35ns max.
DATA N VALID
DATA N+1 VALID
Figure 3. CDS-1401 in Front of ADS-CCD1201 at fCLK = 1MHz
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