LTC2226IUH#PBF

LTC2228/LTC2227/LTC2226 12-Bit, 65/40/25Msps Low Power 3V ADCs FEATURES DESCRIPTION n The LTC®2228/LTC2227/LTC2226 are 12-bit 65Msps/ 40Msps/25Msps, low power 3V A/D converters designed for digitizing high frequency, wide dynamic range signals. The LTC2228/LTC2227/LTC2226 are perfect for demanding imaging and communications applications with AC performance that includes 71.3dB SNR and 90dB SFDR for signals at the Nyquist frequency. n n n n n n n n n n n Sample Rate: 65Msps/40Msps/25Msps Single 3V Supply (2.7V to 3.4V) Low Power: 205mW/120mW/75mW 71.3dB SNR 90dB SFDR No Missing Codes Flexible Input: 1VP-P to 2VP-P Range 575MHz Full Power Bandwidth S/H Clock Duty Cycle Stabilizer Shutdown and Nap Modes Pin Compatible Family 125Msps: LTC2253 (12-Bit), LTC2255 (14-Bit) 105Msps: LTC2252 (12-Bit), LTC2254 (14-Bit) 80Msps: LTC2229 (12-Bit), LTC2249 (14-Bit) 65Msps: LTC2228 (12-Bit), LTC2248 (14-Bit) 40Msps: LTC2227 (12-Bit), LTC2247 (14-Bit) 25Msps: LTC2226 (12-Bit), LTC2246 (14-Bit) 10Msps: LTC2225 (12-Bit), LTC2245 (14-Bit) 32-Pin (5mm × 5mm) QFN Package DC specs include ±0.3LSB INL (typ), ±0.15LSB DNL (typ) and no missing codes over temperature. The transition noise is a low 0.25LSBRMS. A single 3V supply allows low power operation. A separate output supply allows the outputs to drive 0.5V to 3.6V logic. A single-ended CLK input controls converter operation. An optional clock duty cycle stabilizer allows high performance at full speed for a wide range of clock duty cycles. L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. APPLICATIONS n n n n n Wireless and Wired Broadband Communication Imaging Systems Ultrasound Spectral Analysis Portable Instrumentation TYPICAL APPLICATION REFH REFL LTC2228: SNR vs Input Frequency, –1dB, 2V Range, 65Msps 72 FLEXIBLE REFERENCE OVDD ANALOG INPUT INPUT S/H – 12-BIT PIPELINED ADC CORE CORRECTION LOGIC D11 • • • D0 OUTPUT DRIVERS OGND CLOCK/DUTY CYCLE CONTROL 70 69 68 222876 TA01 CLK SNR (dBFS) 71 + 0 100 150 50 INPUT FREQUENCY (MHz) 200 2228 G09 222876fb 1 LTC2228/LTC2227/LTC2226 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION OVDD = VDD (Notes 1, 2) D9 D10 D11 OF MODE SENSE VCM TOP VIEW VDD Supply Voltage (VDD) ..................................................4V Digital Output Ground Voltage (OGND) ........ –0.3V to 1V Analog Input Voltage (Note 3) .......–0.3V to (VDD + 0.3V) Digital Input Voltage......................–0.3V to (VDD + 0.3V) Digital Output Voltage ................ –0.3V to (OVDD + 0.3V) Power Dissipation .............................................1500mW Operating Temperature Range LTC2228C, LTC2227C, LTC2226C............. 0°C to 70°C LTC2228I, LTC2227I, LTC2226I ............ –40°C to 85°C Storage Temperature Range................... –65°C to 125°C 32 31 30 29 28 27 26 25 AIN+ 1 24 D8 AIN– 2 23 D7 REFH 3 22 D6 REFH 4 21 OVDD 33 REFL 5 20 OGND REFL 6 19 D5 VDD 7 18 D4 GND 8 17 D3 D2 D1 D0 NC NC OE CLK SHDN 9 10 11 12 13 14 15 16 UH PACKAGE 32-LEAD (5mm s 5mm) PLASTIC QFN TJMAX = 125°C, θJA = 34°C/W EXPOSED PAD (PIN 33) IS GND, MUST BE SOLDERED TO PCB ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LTC2228CUH#PBF LTC2228CUH#TRPBF 2228 32-Lead (5mm × 5mm) Plastic QFN 0°C to 70°C LTC2228IUH#PBF LTC2228IUH#TRPBF 2228 32-Lead (5mm × 5mm) Plastic QFN –40°C to 85°C LTC2227CUH#PBF LTC2227CUH#TRPBF 2227 32-Lead (5mm × 5mm) Plastic QFN 0°C to 70°C LTC2227IUH#PBF LTC2227IUH#TRPBF 2227 32-Lead (5mm × 5mm) Plastic QFN –40°C to 85°C LTC2226CUH#PBF LTC2226CUH#TRPBF 2226 32-Lead (5mm × 5mm) Plastic QFN 0°C to 70°C LTC2226IUH#PBF LTC2226IUH#TRPBF 2226 32-Lead (5mm × 5mm) Plastic QFN –40°C to 85°C LEAD BASED FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LTC2228CUH LTC2228CUH#TR 2228 32-Lead (5mm × 5mm) Plastic QFN 0°C to 70°C LTC2228IUH LTC2228IUH#TR 2228 32-Lead (5mm × 5mm) Plastic QFN –40°C to 85°C LTC2227CUH LTC2227CUH#TR 2227 32-Lead (5mm × 5mm) Plastic QFN 0°C to 70°C LTC2227IUH LTC2227IUH#TR 2227 32-Lead (5mm × 5mm) Plastic QFN –40°C to 85°C LTC2226CUH LTC2226CUH#TR 2226 32-Lead (5mm × 5mm) Plastic QFN 0°C to 70°C LTC2226IUH LTC2226IUH#TR 2226 32-Lead (5mm × 5mm) Plastic QFN –40°C to 85°C Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ 222876fb 2 LTC2228/LTC2227/LTC2226 CONVERTER CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 4) PARAMETER CONDITIONS MIN Resolution (No Missing Codes) l 12 LTC2228 TYP MAX MIN LTC2227 TYP MAX 12 MIN LTC2226 TYP MAX UNITS 12 Bits Integral Linearity Error Differential Analog Input (Note 5) l –1.1 ±0.3 1.1 –1 ±0.3 1 –1 ±0.3 1 LSB Differential Linearity Error Differential Analog Input l –0.8 ±0.15 0.8 –0.7 ±0.15 0.7 –0.7 ±0.15 0.7 LSB Offset Error (Note 6) l –12 ±2 12 –12 ±2 12 –12 ±2 12 mV Gain Error External Reference l –2.5 ±0.5 2.5 –2.5 ±0.5 2.5 –2.5 ±0.5 2.5 Offset Drift Full-Scale Drift Internal Reference External Reference Transition Noise SENSE = 1V %FS ±10 ±10 ±10 μV/°C ±30 ±30 ±30 ppm/°C ±5 ±5 ±5 ppm/°C 0.25 0.25 0.25 LSBRMS ANALOG INPUT The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 4) SYMBOL PARAMETER CONDITIONS VIN Analog Input Range (AIN+ – AIN–) 2.7V < VDD < 3.4V (Note 7) l MIN TYP MAX UNITS VIN,CM Analog Input Common Mode (AIN+ + AIN–)/2 Differential Input (Note 7) Single-Ended Input (Note 7) l l 1 0.5 IIN Analog Input Leakage Current 0V < AIN+, AIN– < VDD l ISENSE SENSE Input Leakage 0V < SENSE < 1V IMODE MODE Pin Leakage tAP Sample-and-Hold Acquisition Delay Time tJITTER Sample-and-Hold Acquisition Delay Time Jitter 0.2 psRMS CMRR Analog Input Common Mode Rejection Ratio 80 dB 575 MHz ±0.5V to ±1V 1.5 1.5 V 1.9 2 V V –1 1 μA l –3 3 μA l –3 3 μA 0 Full Power Bandwidth Figure 8 Test Circuit ns DYNAMIC ACCURACY The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. AIN = –1dBFS. (Note 4) SYMBOL PARAMETER CONDITIONS SNR 5MHz Input 12.5MHz Input 20MHz Input 30MHz Input 70MHz Input 140MHz Input 5MHz Input 12.5MHz Input 20MHz Input 30MHz Input 70MHz Input 140MHz Input SFDR Signal-to-Noise Ratio Spurious Free Dynamic Range 2nd or 3rd Harmonic MIN LTC2228 TYP MAX MIN 71.3 LTC2227 TYP MAX 71.4 l l l 70.1 70 71.3 71.3 71 90 l 75 90 85 80 70.2 71.4 71.2 76 70.9 70.6 90 90 71.1 70.7 90 76 LTC2226 TYP MAX 71.3 l l MIN 90 85 80 85 80 UNITS dB dB dB dB dB dB dB dB dB dB dB dB 222876fb 3 LTC2228/LTC2227/LTC2226 DYNAMIC ACCURACY The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. AIN = –1dBFS. (Note 4) SYMBOL PARAMETER CONDITIONS SFDR 5MHz Input 12.5MHz Input 20MHz Input 30MHz Input 70MHz Input 140MHz Input 5MHz Input 12.5MHz Input 20MHz Input 30MHz Input 70MHz Input 140MHz Input fIN1 = 28.2MHz, fIN2 = 26.8MHz S/(N+D) IMD Spurious Free Dynamic Range 4th Harmonic or Higher Signal-to-Noise Plus Distortion Ratio Intermodulation Distortion MIN LTC2228 TYP MAX MIN LTC2227 TYP MAX 95 95 l l l 82 82 95 95 90 71.3 l 82 95 95 69.8 95 90 71.4 71.2 95 90 71.4 69.7 69.6 71.2 71.1 69.9 90 LTC2226 TYP MAX 71.2 70.9 69.9 90 UNITS dB dB dB dB dB dB dB dB dB dB dB dB dB 95 l l MIN 70.8 69.8 90 INTERNAL REFERENCE CHARACTERISTICS (Note 4) PARAMETER VCM Output Voltage VCM Output Tempco VCM Line Regulation VCM Output Resistance CONDITIONS IOUT = 0 MIN 1.475 TYP 1.500 ±25 3 4 2.7V < VDD < 3.4V –1mA < IOUT < 1mA MAX 1.525 UNITS V ppm/°C mV/V Ω DIGITAL INPUTS AND DIGITAL OUTPUTS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 4) SYMBOL PARAMETER LOGIC INPUTS (CLK, OE, SHDN) High Level Input Voltage VIH Low Level Input Voltage VIL Input Current IIN Input Capacitance CIN LOGIC OUTPUTS OVDD = 3V Hi-Z Output Capacitance COZ Output Source Current ISOURCE Output Sink Current ISINK High Level Output Voltage VOH VOL Low Level Output Voltage OVDD = 2.5V High Level Output Voltage VOH Low Level Output Voltage VOL OVDD = 1.8V High Level Output Voltage VOH Low Level Output Voltage VOL CONDITIONS VDD = 3V VDD = 3V VIN = 0V to VDD (Note 7) OE = High (Note 7) VOUT = 0V VOUT = 3V IO = –10μA IO = –200μA IO = 10μA IO = 1.6mA MIN l TYP 2 l 3 3 50 50 2.995 2.99 0.005 0.09 pF mA mA V V V V l 0.8 10 –10 2.7 UNITS V V μA pF l l MAX 0.4 IO = –200μA IO = 1.6mA 2.49 0.09 V V IO = –200μA IO = 1.6mA 1.79 0.09 V V 222876fb 4 LTC2228/LTC2227/LTC2226 POWER REQUIREMENTS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 8) SYMBOL PARAMETER CONDITIONS MIN LTC2228 TYP MAX LTC2227 TYP MAX MIN MIN LTC2226 TYP MAX UNITS VDD Analog Supply Voltage (Note 9) l 2.7 3 3.4 2.7 3 3.4 2.7 3 3.4 V OVDD Output Supply Voltage (Note 9) l 0.5 3 3.6 0.5 3 3.6 0.5 3 3.6 V IVDD Supply Current l 68.3 80 40 48 25 30 mA PDISS Power Dissipation l 205 240 120 144 75 90 PSHDN Shutdown Power SHDN = H, OE = H, No CLK 2 2 2 mW PNAP Nap Mode Power SHDN = H, OE = L, No CLK 15 15 15 mW mW TIMING CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 4) SYMBOL PARAMETER CONDITIONS MIN fS Sampling Frequency (Note 9) l 1 tL CLK Low Time Duty Cycle Stabilizer Off Duty Cycle Stabilizer On (Note 7) l l 7.3 5 tH CLK High Time Duty Cycle Stabilizer Off Duty Cycle Stabilizer On (Note 7) l l 7.3 5 tAP Sample-and-Hold Aperture Delay tD CLK to DATA Delay CL = 5pF (Note 7) l Data Access Time After OE↓ CL = 5pF (Note 7) BUS Relinquish Time (Note 7) LTC2228 TYP MAX MIN 65 1 7.7 7.7 500 500 11.8 5 7.7 7.7 500 500 11.8 5 0 1.4 5.4 l 4.3 l 3.3 Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: All voltage values are with respect to ground with GND and OGND wired together (unless otherwise noted). Note 3: When these pin voltages are taken below GND or above VDD, they will be clamped by internal diodes. This product can handle input currents of greater than 100mA below GND or above VDD without latchup. Note 4: VDD = 3V, fSAMPLE = 65MHz (LTC2228), 40MHz (LTC2227), or 25MHz (LTC2226), input range = 2VP-P with differential drive, unless otherwise noted. 5 MIN 40 1 12.5 500 500 18.9 5 12.5 12.5 500 500 18.9 5 0 2.7 Pipeline Latency LTC2227 TYP MAX 1.4 UNITS 25 MHz 20 20 500 500 ns ns 20 20 500 500 ns ns 0 2.7 5.4 10 4.3 8.5 3.3 5 LTC2226 TYP MAX 1.4 ns 2.7 5.4 ns 10 4.3 10 ns 8.5 3.3 8.5 5 ns Cycles Note 5: Integral nonlinearity is defined as the deviation of a code from a straight line passing through the actual endpoints of the transfer curve. The deviation is measured from the center of the quantization band. Note 6: Offset error is the offset voltage measured from –0.5 LSB when the output code flickers between 0000 0000 0000 and 1111 1111 1111. Note 7: Guaranteed by design, not subject to test. Note 8: VDD = 3V, fSAMPLE = 65MHz (LTC2228), 40MHz (LTC2227), or 25MHz (LTC2226), input range = 1VP-P with differential drive. Note 9: Recommend operating conditions. 222876fb 5 LTC2228/LTC2227/LTC2226 TYPICAL PERFORMANCE CHARACTERISTICS LTC2228: Typical INL, 2V Range, 65Msps 0 1.00 0.75 0.50 0.50 0.25 0 –0.25 –0.50 –10 –20 –30 AMPLITUDE (dB) 0.75 DNL ERROR (LSB) INL ERROR (LSB) 1.00 0.25 0 –0.25 –0.75 –1.00 –1.00 1024 0 2048 CODE 3072 4096 –50 –60 –70 –80 –100 –110 –120 0 1024 2048 CODE 3072 2228 G01 4096 –20 –20 –20 –30 –30 –30 –70 –80 AMPLITUDE (dB) 0 –10 AMPLITUDE (dB) 0 –10 –60 –40 –50 –60 –70 –80 –50 –60 –70 –80 –90 –90 –100 –100 –100 –110 –110 –110 –120 –120 5 10 15 20 25 FREQUENCY (MHz) 30 0 5 0 5 10 15 20 25 FREQUENCY (MHz) 2228 G05 LTC2228: 8192 Point 2-Tone FFT, fIN = 28.2MHz and 26.8MHz, –1dB, 2V Range, 65Msps LTC2228: SNR vs Input Frequency, –1dB, 2V Range, 65Msps 70000 0 30 2228 G06 LTC2228: Grounded Input Histogram, 65Msps 72 –10 61496 60000 –20 –30 71 COUNT –50 –60 –70 –80 SNR (dBFS) 50000 –40 40000 30000 20000 –90 –100 70 69 10000 –110 –120 –120 30 10 15 20 25 FREQUENCY (MHz) 2228 G04 30 –40 –90 0 10 15 20 25 FREQUENCY (MHz) 2228 G03 0 –50 5 LTC2228: 8192 Point FFT, fIN = 140MHz, –1dB, 2V Range, 65Msps –10 –40 0 2228 G02 LTC2228: 8192 Point FFT, fIN = 70MHz, –1dB, 2V Range, 65Msps LTC2228: 8192 Point FFT, fIN = 30MHz, –1dB, 2V Range, 65Msps AMPLITUDE (dB) –40 –90 –0.50 –0.75 AMPLITUDE (dB) LTC2228: 8192 Point FFT, fIN = 5MHz, –1dB, 2V Range, 65Msps LTC2228: Typical DNL, 2V Range, 65Msps 2123 0 5 10 15 20 25 FREQUENCY (MHz) 30 2228 G07 1910 0 2042 2043 CODE 2044 2228 G08 68 0 100 150 50 INPUT FREQUENCY (MHz) 200 2228 G09 222876fb 6 LTC2228/LTC2227/LTC2226 TYPICAL PERFORMANCE CHARACTERISTICS LTC2228: SNR and SFDR vs Sample Rate, 2V Range,fIN = 5MHz, –1dB LTC2228: SFDR vs Input Frequency, –1dB, 2V Range, 65Msps 100 LTC2228: SNR and SFDR vs Clock Duty Cycle, 65Msps 110 100 95 95 85 80 75 SFDR 90 80 SNR SFDR: DCS ON SFDR: DCS OFF 90 85 80 75 SNR: DCS ON 70 70 70 65 50 100 150 INPUT FREQUENCY (MHz) 200 60 20 0 60 80 40 SAMPLE RATE (Msps) 65 100 2228 G10 SNR: DCS OFF 30 35 45 50 55 60 CLOCK DUTY CYCLE (%) 40 2228 G11 65 70 2228 G12 LTC2228: SFDR vs Input Level, fIN = 30MHz, 2V Range, 65Msps LTC2228: SNR vs Input Level, fIN = 30MHz, 2V Range, 65Msps 120 80 dBFS 110 70 dBFS 100 60 50 SFDR (dBc AND dBFS) SNR (dBc AND dBFS) dBc 40 30 20 90 80 dBc 70 60 90dBc SFDR REFERENCE LINE 50 40 10 30 0 –60 –50 –40 –30 –20 INPUT LEVEL (dBFS) –10 20 –60 0 –50 –40 –30 –20 INPUT LEVEL (dBFS) 2228 G13 80 6 75 5 70 4 2V RANGE 65 1V RANGE 3 60 2 55 1 0 10 20 30 40 50 60 SAMPLE RATE (Msps) 70 0 LTC2228: IOVDD vs Sample Rate, 5MHz Sine Wave Input, –1dB, OVDD = 1.8V LTC2228: IVDD vs Sample Rate, 5mHz Sine Wave Input, –1dB 50 –10 2228 G14 IOVDD (mA) 0 IVDD (mA) SFDR (dBFS) 90 SNR AND SFDR (dBFS) SNR AND SFDR (dBFS) 100 80 2228 G15 0 0 10 20 30 40 50 60 SAMPLE RATE (Msps) 70 80 2228 G16 222876fb 7 LTC2228/LTC2227/LTC2226 TYPICAL PERFORMANCE CHARACTERISTICS LTC2227: Typical INL, 2V Range, 40Msps 0 1.00 0.75 0.50 0.50 0.25 0 –0.25 –0.50 –10 –20 –30 AMPLITUDE (dB) 0.75 DNL ERROR (LSB) INL ERROR (LSB) 1.00 0.25 0 –0.25 –0.50 –0.75 –0.75 –1.00 –1.00 0 1024 2048 CODE 3072 –50 –60 –70 –80 –90 –110 1024 2048 CODE 3072 2227 G01 –120 4096 0 –10 –20 –20 –20 –30 –30 –30 –70 –80 AMPLITUDE (dB) 0 –10 –60 –40 –50 –60 –70 –80 –50 –60 –70 –80 –90 –90 –100 –100 –100 –110 –110 –110 –120 –120 5 10 15 FREQUENCY (MHz) 20 0 5 10 15 FREQUENCY (MHz) 2227 G04 0 5 10 15 FREQUENCY (MHz) 0 20 2227 G06 LTC2227: Grounded Input Histogram, 40Msps LTC2227: SNR vs Input Frequency, –1dB, 2V Range, 40Msps 72 70000 –10 61538 60000 –20 –30 71 COUNT –50 –60 –70 –80 SNR (dBFS) 50000 –40 40000 30000 20000 –90 –100 70 69 10000 –110 –120 –120 20 2227 G05 LTC2227: 8192 Point 2-Tone FFT, fIN = 21.6MHz and 23.6MHz, –1dB, 2V Range, 40Msps 20 –40 –90 0 10 15 FREQUENCY (MHz) 2227 G03 0 –50 5 LTC2227: 8192 Point FFT, fIN = 140MHz, –1dB, 2V Range, 40Msps –10 –40 0 2227 G02 LTC2227: 8192 Point FFT, fIN = 70MHz, –1dB, 2V Range, 40Msps AMPLITUDE (dB) AMPLITUDE (dB) –40 –100 0 4096 LTC2228: 8192 Point FFT, fIN = 30MHz, –1dB, 2V Range, 40Msps AMPLITUDE (dB) LTC2227: 8192 Point FFT, fIN = 5MHz, –1dB, 2V Range, 40Msps LTC2227: Typical DNL, 2V Range, 40Msps 2558 1424 0 0 5 10 15 FREQUENCY (MHz) 20 2227 G07 68 2050 2051 CODE 2052 2227 G08 0 100 150 50 INPUT FREQUENCY (MHz) 200 2227 G09 222876fb 8 LTC2228/LTC2227/LTC2226 TYPICAL PERFORMANCE CHARACTERISTICS LTC2227: SFDR vs Input Frequency, –1dB, 2V Range, 40Msps LTC2227: SNR and SFDR vs Sample Rate, 2V Range,fIN = 5MHz, –1dB 110 100 80 dBFS 95 85 80 75 SNR (dBc AND dBFS) SNR AND SFDR (dBFS) 90 70 SFDR 100 SFDR (dBFS) LTC2227: SNR vs Input Level, fIN = 5MHz, 2V Range, 40Msps 90 80 SNR 60 50 dBc 40 30 20 70 70 10 65 0 –60 60 0 50 100 0 200 150 INPUT FREQUENCY (MHz) 20 40 60 SAMPLE RATE (Msps) 80 –50 –40 –30 –20 INPUT LEVEL (dBFS) –10 2227 G12 2227 G11 2227 G10 LTC2227: SFDR vs Input Level, fIN = 5MHz, 2V Range, 40Msps LTC2227: IOVDD vs Sample Rate, 5MHz Sine Wave Input, –1dB, OVDD = 1.8V LTC2227: IVDD vs Sample Rate, 5MHz Sine Wave Input, –1dB 120 0 50 4 45 3 110 dBFS 80 dBc 70 60 90dBc SFDR REFERENCE LINE 50 2V RANGE IOVDD (mA) 90 IVDD (mA) SNR (dBc AND dBFS) 100 40 1V RANGE 35 2 1 40 30 20 –60 30 –50 –40 –30 –20 –10 0 0 10 20 30 40 50 SAMPLE RATE (Msps) INPUT LEVEL (dBFS) 2227 G13 0 0 10 20 30 40 50 SAMPLE RATE (Msps) 2227 G14 2227 G15 222876fb 9 LTC2228/LTC2227/LTC2226 TYPICAL PERFORMANCE CHARACTERISTICS LTC2226: Typical INL, 2V Range, 25Msps 0 1.00 0.75 0.50 0.50 0.25 0 –0.25 –0.50 –10 –20 –30 AMPLITUDE (dB) 0.75 DNL ERROR (LSB) INL ERROR (LSB) 1.00 0.25 0 –0.25 –0.75 –1.00 –1.00 1024 0 2048 CODE 3072 4096 –50 –60 –70 –80 –90 –100 –110 1024 0 2048 CODE 3072 2226 G01 –120 4096 0 –10 –20 –20 –20 –30 –30 –30 –70 –80 AMPLITUDE (dB) 0 –10 AMPLITUDE (dB) 0 –60 –40 –50 –60 –70 –80 –50 –60 –70 –80 –90 –90 –100 –100 –100 –110 –110 –110 –120 –120 2 4 6 8 FREQUENCY (MHz) 10 12 0 2 4 6 8 FREQUENCY (MHz) 2226 G04 12 10 –120 0 2 4 6 8 FREQUENCY (MHz) 10 2226 G05 LTC2226: 8192 Point 2-Tone FFT, fIN = 10.9MHz and 13.8MHz, –1dB, 2V Range, 25Msps 12 2226 G06 LTC2226: Grounded Input Histogram, 25Msps 0 12 10 –40 –90 0 4 6 8 FREQUENCY (MHz) LTC2226: 8192 Point FFT, fIN = 140MHz, –1dB, 2V Range, 25Msps –10 –50 2 2226 G03 LTC2226: 8192 Point FFT, fIN = 70MHz, –1dB, 2V Range, 25Msps –40 0 2226 G02 LTC2226: 8192 Point FFT, fIN = 30MHz, –1dB, 2V Range, 25Msps AMPLITUDE (dB) –40 –0.50 –0.75 LTC2226: SNR vs Input Frequency, –1dB, 2V Range, 25Msps 72 70000 61758 –10 60000 –20 71 –30 –50 –60 –70 SNR (dBFS) 50000 –40 COUNT AMPLITUDE (dB) LTC2226: 8192 Point FFT, fIN = 5MHz, –1dB, 2V Range, 25Msps LTC2226: Typical DNL, 2V Range, 25Msps 40000 30000 70 –80 20000 –90 –100 10000 –110 –120 69 2155 0 2 4 6 8 FREQUENCY (MHz) 10 12 2226 G07 10 0 1607 68 2048 2049 CODE 2050 2226 G08 0 100 150 50 INPUT FREQUENCY (MHz) 200 2226 G09 222876fb LTC2228/LTC2227/LTC2226 TYPICAL PERFORMANCE CHARACTERISTICS LTC2226: SFDR vs Input Frequency, –1dB, 2V Range, 25Msps LTC2226: SNR and SFDR vs Sample Rate, 2V Range,fIN = 5MHz, –1dB 100 LTC2226: SNR vs Input Level, fIN = 5MHz, 2V Range, 25Msps 110 80 dBFS 95 SFDR (dBFS) 85 80 75 SNR (dBc AND dBFS) SNR AND SFDR (dBFS) 100 90 70 SFDR 90 80 SNR 50 dBc 40 30 20 70 70 10 65 50 100 60 200 150 INPUT FREQUENCY (MHz) 0 10 0 30 40 20 SAMPLE RATE (Msps) 2226 G10 0 –60 50 –50 –40 –30 –20 INPUT LEVEL (dBFS) –10 LTC2226: IOVDD vs Sample Rate, 5MHz Sine Wave Input, –1dB, OVDD = 1.8V LTC2226: IVDD vs Sample Rate, 5MHz Sine Wave Input, –1dB 35 120 0 2227 G12 2226 G11 LTC2226: SFDR vs Input Level, fIN = 5MHz, 2V Range, 25Msps 3 dBFS 110 100 30 90 2V RANGE dBc 70 60 90dBc SFDR REFERENCE LINE 50 2 IOVDD (mA) 80 IVDD (mA) SFDR (dBc AND dBFS) 60 25 1V RANGE 1 20 40 30 20 –60 15 –50 –40 –30 –20 INPUT LEVEL (dBFS) –10 0 2226 G13 0 5 10 15 20 25 SAMPLE RATE (Msps) 30 35 2226 G14 0 0 5 10 15 25 20 SAMPLE RATE (Msps) 30 35 2226 G15 222876fb 11 LTC2228/LTC2227/LTC2226 PIN FUNCTIONS AIN+ (Pin 1): Positive Differential Analog Input. NC (Pins 12, 13): Do Not Connect These Pins. AIN– (Pin 2): Negative Differential Analog Input. D0-D11 (Pins 14, 15, 16, 17, 18, 19, 22, 23, 24, 25, 26, 27): Digital Outputs. D11 is the MSB. REFH (Pins 3, 4): ADC High Reference. Short together and bypass to Pins 5, 6 with a 0.1μF ceramic chip capacitor as close to the pin as possible. Also bypass to Pins 5, 6 with an additional 2.2μF ceramic chip capacitor and to ground with a 1μF ceramic chip capacitor. REFL (Pins 5, 6): ADC Low Reference. Short together and bypass to Pins 3, 4 with a 0.1μF ceramic chip capacitor as close to the pin as possible. Also bypass to Pins 3, 4 with an additional 2.2μF ceramic chip capacitor and to ground with a 1μF ceramic chip capacitor. VDD (Pins 7, 32): 3V Supply. Bypass to GND with 0.1μF ceramic chip capacitors. GND (Pin 8): ADC Power Ground. CLK (Pin 9): Clock Input. The input sample starts on the positive edge. SHDN (Pin 10): Shutdown Mode Selection Pin. Connecting SHDN to GND and OE to GND results in normal operation with the outputs enabled. Connecting SHDN to GND and OE to VDD results in normal operation with the outputs at high impedance. Connecting SHDN to VDD and OE to GND results in nap mode with the outputs at high impedance. Connecting SHDN to VDD and OE to VDD results in sleep mode with the outputs at high impedance. OE (Pin 11): Output Enable Pin. Refer to SHDN pin function. OGND (Pin 20): Output Driver Ground. OVDD (Pin 21): Positive Supply for the Output Drivers. Bypass to ground with 0.1μF ceramic chip capacitor. OF (Pin 28): Over/Under Flow Output. High when an over or under flow has occurred. MODE (Pin 29): Output Format and Clock Duty Cycle Stabilizer Selection Pin. Connecting MODE to GND selects offset binary output format and turns the clock duty cycle stabilizer off. 1/3 VDD selects offset binary output format and turns the clock duty cycle stabilizer on. 2/3 VDD selects 2’s complement output format and turns the clock duty cycle stabilizer on. VDD selects 2’s complement output format and turns the clock duty cycle stabilizer off. SENSE (Pin 30): Reference Programming Pin. Connecting SENSE to VCM selects the internal reference and a ±0.5V input range. VDD selects the internal reference and a ±1V input range. An external reference greater than 0.5V and less than 1V applied to SENSE selects an input range of ±VSENSE. ±1V is the largest valid input range. VCM (Pin 31): 1.5V Output and Input Common Mode Bias. Bypass to ground with 2.2μF ceramic chip capacitor. Exposed Pad (Pin 33): ADC Power Ground. The Exposed Pad on the bottom of the package needs to be soldered to ground. 222876fb 12 LTC2228/LTC2227/LTC2226 FUNCTIONAL BLOCK DIAGRAM AIN+ AIN– VCM INPUT S/H FIRST PIPELINED ADC STAGE SECOND PIPELINED ADC STAGE THIRD PIPELINED ADC STAGE FOURTH PIPELINED ADC STAGE FIFTH PIPELINED ADC STAGE 1.5V REFERENCE SIXTH PIPELINED ADC STAGE SHIFT REGISTER AND CORRECTION 2.2μF RANGE SELECT REFH SENSE REFL INTERNAL CLOCK SIGNALS OVDD REF BUF OF D11 CLOCK/DUTY CYCLE CONTROL DIFF REF AMP CONTROL LOGIC OUTPUT DRIVERS • • • D0 REFH 0.1μF 222876 F01 REFL OGND CLK M0DE SHDN OE 2.2μF 1μF 1μF Figure 1. Functional Block Diagram 222876fb 13 LTC2228/LTC2227/LTC2226 TIMING DIAGRAM Timing Diagram tAP ANALOG INPUT N+4 N+2 N N+3 tH N+5 N+1 tL CLK tD D0-D11, OF N–5 N–4 N–3 N–2 N–1 N 222876 TD01 222876fb 14 LTC2228/LTC2227/LTC2226 APPLICATIONS INFORMATION DYNAMIC PERFORMANCE Signal-to-Noise Plus Distortion Ratio The signal-to-noise plus distortion ratio [S/(N + D)] is the ratio between the RMS amplitude of the fundamental input frequency and the RMS amplitude of all other frequency components at the ADC output. The output is band limited to frequencies above DC to below half the sampling frequency. Signal-to-Noise Ratio The signal-to-noise ratio (SNR) is the ratio between the RMS amplitude of the fundamental input frequency and the RMS amplitude of all other frequency components except the first five harmonics and DC. Total Harmonic Distortion Total harmonic distortion is the ratio of the RMS sum of all harmonics of the input signal to the fundamental itself. The out-of-band harmonics alias into the frequency band between DC and half the sampling frequency. THD is expressed as: THD = 20Log (√(V22 + V32 + V42 + . . . Vn2)/V1) where V1 is the RMS amplitude of the fundamental frequency and V2 through Vn are the amplitudes of the second through nth harmonics. The THD calculated in this data sheet uses all the harmonics up to the fifth. Intermodulation Distortion If the ADC input signal consists of more than one spectral component, the ADC transfer function nonlinearity can produce intermodulation distortion (IMD) in addition to THD. IMD is the change in one sinusoidal input caused by the presence of another sinusoidal input at a different frequency. If two pure sine waves of frequencies fa and fb are applied to the ADC input, nonlinearities in the ADC transfer function can create distortion products at the sum and difference frequencies of mfa ± nfb, where m and n = 0, 1, 2, 3, etc. The 3rd order intermodulation products are 2fa + fb, 2fb + fa, 2fa – fb and 2fb – fa. The intermodulation distortion is defined as the ratio of the RMS value of either input tone to the RMS value of the largest 3rd order intermodulation product. Spurious Free Dynamic Range (SFDR) Spurious free dynamic range is the peak harmonic or spurious noise that is the largest spectral component excluding the input signal and DC. This value is expressed in decibels relative to the RMS value of a full-scale input signal. Input Bandwidth The input bandwidth is that input frequency at which the amplitude of the reconstructed fundamental is reduced by 3dB for a full-scale input signal. Aperture Delay Time The time from when CLK reaches mid-supply to the instant that the input signal is held by the sample-and-hold circuit. Aperture Delay Jitter The variation in the aperture delay time from conversion to conversion. This random variation will result in noise when sampling an AC input. The signal to noise ratio due to the jitter alone will be: SNRJITTER = –20log (2π • fIN • tJITTER) 222876fb 15 LTC2228/LTC2227/LTC2226 APPLICATIONS INFORMATION CONVERTER OPERATION SAMPLE/HOLD OPERATION AND INPUT DRIVE As shown in Figure 1, the LTC2228/LTC2227/LTC2226 is a CMOS pipelined multi-step converter. The converter has six pipelined ADC stages; a sampled analog input will result in a digitized value five cycles later (see the Timing Diagram section). For optimal AC performance the analog inputs should be driven differentially. For cost sensitive applications, the analog inputs can be driven single-ended with slightly worse harmonic distortion. The CLK input is single-ended. The LTC2228/LTC2227/LTC2226 has two phases of operation, determined by the state of the CLK input pin. Sample/Hold Operation Each pipelined stage shown in Figure 1 contains an ADC, a reconstruction DAC and an interstage residue amplifier. In operation, the ADC quantizes the input to the stage and the quantized value is subtracted from the input by the DAC to produce a residue. The residue is amplified and output by the residue amplifier. Successive stages operate out of phase so that when the odd stages are outputting their residue, the even stages are acquiring that residue and vice versa. When CLK is low, the analog input is sampled differentially directly onto the input sample-and-hold capacitors, inside the “Input S/H” shown in the Block Diagram. At the instant that CLK transitions from low to high, the sampled input is held. While CLK is high, the held input voltage is buffered by the S/H amplifier which drives the first pipelined ADC stage. The first stage acquires the output of the S/H during this high phase of CLK. When CLK goes back low, the first stage produces its residue which is acquired by the second stage. At the same time, the input S/H goes back to acquiring the analog input. When CLK goes back high, the second stage produces its residue which is acquired by the third stage. An identical process is repeated for the third, fourth and fifth stages, resulting in a fifth stage residue that is sent to the sixth stage ADC for final evaluation. Each ADC stage following the first has additional range to accommodate flash and amplifier offset errors. Results from all of the ADC stages are digitally synchronized such that the results can be properly combined in the correction logic before being sent to the output buffer. Figure 2 shows an equivalent circuit for the LTC2228/ LTC2227/LTC2226 CMOS differential sample-and-hold. The analog inputs are connected to the sampling capacitors (CSAMPLE) through NMOS transistors. The capacitors shown attached to each input (CPARASITIC) are the summation of all other capacitance associated with each input. During the sample phase when CLK is low, the transistors connect the analog inputs to the sampling capacitors and they charge to and track the differential input voltage. When CLK transitions from low to high, the sampled input voltage is held on the sampling capacitors. During the hold phase when CLK is high, the sampling capacitors are disconnected from the input and the held voltage is passed to the ADC core for processing. As CLK transitions from high to low, the inputs are reconnected to the sampling capacitors to acquire a new sample. Since the sampling capacitors still hold the previous sample, a charging glitch proportional to the change in voltage between samples will be seen at this time. If the change between the last sample and the new sample is small, the charging glitch seen at the input will be small. If the input change is large, such as the change seen with input frequencies near Nyquist, then a larger charging glitch will be seen. LTC2228/27/26 VDD AIN+ CSAMPLE 4pF 15Ω CPARASITIC 1pF VDD AIN– CSAMPLE 4pF 15Ω CPARASITIC 1pF VDD CLK 222876 F02 Figure 2. Equivalent Input Circuit 222876fb 16 LTC2228/LTC2227/LTC2226 APPLICATIONS INFORMATION however, this is not always possible and the incomplete settling may degrade the SFDR. The sampling glitch has been designed to be as linear as possible to minimize the effects of incomplete settling. Single-Ended Input For cost-sensitive applications, the analog inputs can be driven single ended. With a single-ended input the harmonic distortion and INL will degrade, but the SNR and DNL will remain unchanged. For a single-ended input, AIN+ should be driven with the input signal and AIN– should be connected to 1.5V or VCM. For the best performance, it is recommended to have a source impedance of 100Ω or less for each input. The source impedance should be matched for the differential inputs. Poor matching will result in higher even order harmonics, especially the second. Common Mode Bias For optimal performance the analog inputs should be driven differentially. Each input should swing ±0.5V for the 2V range or ±0.25V for the 1V range, around a common mode voltage of 1.5V. The VCM output pin (Pin 31) may be used to provide the common mode bias level. VCM can be tied directly to the center tap of a transformer to set the DC input level or as a reference level to an op amp differential driver circuit. The VCM pin must be bypassed to ground close to the ADC with a 2.2μF or greater capacitor. Input Drive Circuits Figure 3 shows the LTC2228/LTC2227/LTC2226 being driven by an RF transformer with a center tapped secondary. The secondary center tap is DC biased with VCM, setting the ADC input signal at its optimum DC level. Terminating on the transformer secondary is desirable, as this provides a common mode path for charging glitches caused by the sample and hold. Figure 3 shows a 1:1 turns ratio transformer. Other turns ratios can be used if the source impedance seen by the ADC does not exceed 100Ω for each ADC input. A disadvantage of using a transformer is the loss of low frequency response. Most small RF transformers have poor performance at frequencies below 1MHz. Input Drive Impedance As with all high performance, high speed ADCs, the dynamic performance of the LTC2228/LTC2227/LTC2226 can be influenced by the input drive circuitry, particularly the second and third harmonics. Source impedance and input reactance can influence SFDR. At the falling edge of CLK, the sample-and-hold circuit will connect the 4pF sampling capacitor to the input pin and start the sampling period. The sampling period ends when CLK rises, holding the sampled input on the sampling capacitor. Ideally the input circuitry should be fast enough to fully charge the sampling capacitor during the sampling period 1/(2FENCODE); Figure 4 demonstrates the use of a differential amplifier to convert a single-ended input signal into a differential input signal. The advantage of this method is that it provides low frequency input response; however, the limited gain bandwidth of most op amps will limit the SFDR at high input frequencies. VCM 2.2μF 0.1μF ANALOG INPUT T1 1:1 25Ω 25Ω AIN+ LTC2228/27/26 0.1μF 12pF 25Ω 25Ω T1 = MA/COM ETC1-1T RESISTORS, CAPACITORS ARE 0402 PACKAGE SIZE AIN– 222876 F03 Figure 3. Single-Ended to Differential Conversion Using a Transformer 222876fb 17 LTC2228/LTC2227/LTC2226 APPLICATIONS INFORMATION VCM HIGH SPEED DIFFERENTIAL 25Ω AMPLIFIER ANALOG INPUT + 2.2μF AIN+ 2.2μF 0.1μF LTC2228/27/26 LTC2228/27/26 25Ω 25Ω 0.1μF T1 12pF – 0.1μF AIN– AIN+ 12Ω ANALOG INPUT + CM – VCM 8pF 25Ω AIN– 12Ω T1 = MA/COM, ETC 1-1-13 RESISTORS, CAPACITORS ARE 0402 PACKAGE SIZE 222876 F04 222876 F06 Figure 4. Differential Drive with an Amplifier Figure 5 shows a single-ended input circuit. The impedance seen by the analog inputs should be matched. This circuit is not recommended if low distortion is required. The 25Ω resistors and 12pF capacitor on the analog inputs serve two purposes: isolating the drive circuitry from the sample-and-hold charging glitches and limiting the wideband noise at the converter input. Figure 6. Recommended Front-End Circuit for Input Frequencies Between 70MHz and 170MHz VCM 2.2μF 0.1μF AIN+ ANALOG INPUT 25Ω LTC2228/27/26 0.1μF T1 0.1μF 25Ω AIN– VCM 1k 0.1μF ANALOG INPUT 1k T1 = MA/COM, ETC 1-1-13 RESISTORS, CAPACITORS ARE 0402 PACKAGE SIZE 2.2μF AIN+ 25Ω LTC2228/27/26 Figure 7. Recommended Front-End Circuit for Input Frequencies Between 170MHz and 300MHz 12pF 25Ω 222876 F07 AIN– 0.1μF VCM 222876 F05 2.2μF 0.1μF Figure 5. Single-Ended Drive 6.8nH ANALOG INPUT 25Ω For input frequencies above 70MHz, the input circuits of Figure 6, 7 and 8 are recommended. The balun transformer gives better high frequency response than a flux coupled center tapped transformer. The coupling capacitors allow the analog inputs to be DC biased at 1.5V. In Figure 8, the series inductors are impedance matching elements that maximize the ADC bandwidth. AIN+ LTC2228/27/26 0.1μF T1 0.1μF 25Ω 6.8nH AIN– T1 = MA/COM, ETC 1-1-13 RESISTORS, CAPACITORS, INDUCTORS ARE 0402 PACKAGE SIZE 222876 F08 Figure 8. Recommended Front-End Circuit for Input Frequencies Above 300MHz 222876fb 18 LTC2228/LTC2227/LTC2226 TYPICAL APPLICATIONS Reference Operation Figure 9 shows the LTC2228/LTC2227/LTC2226 reference circuitry consisting of a 1.5V bandgap reference, a difference amplifier and switching and control circuit. The internal voltage reference can be configured for two pin selectable input ranges of 2V (±1V differential) or 1V (±0.5V differential). Tying the SENSE pin to VDD selects the 2V range; tying the SENSE pin to VCM selects the 1V range. The 1.5V bandgap reference serves two functions: its output provides a DC bias point for setting the common mode voltage of any external input circuitry; additionally, the reference is used with a difference amplifier to generate the differential reference levels needed by the internal ADC circuitry. An external bypass capacitor is required for the 1.5V reference output, VCM. This provides a high frequency low impedance path to ground for internal and external circuitry. The difference amplifier generates the high and low reference for the ADC. High speed switching circuits are connected to these outputs and they must be externally bypassed. Each output has two pins. The multiple output pins are needed to reduce package inductance. Bypass capacitors must be connected as shown in Figure 9. Other voltage ranges in-between the pin selectable ranges can be programmed with two external resistors as shown in Figure 10. An external reference can be used by applying its output directly or through a resistor divider to SENSE. It is not recommended to drive the SENSE pin with a logic device. The SENSE pin should be tied to the appropriate level as close to the converter as possible. If the SENSE pin is driven externally, it should be bypassed to ground as close to the device as possible with a 1μF ceramic capacitor. LTC2228/27/26 4Ω VCM 1.5V 1.5V BANDGAP REFERENCE 2.2μF 1V 0.5V RANGE DETECT AND CONTROL TIE TO VDD FOR 2V RANGE; TIE TO VCM FOR 1V RANGE; RANGE = 2 • VSENSE FOR 0.5V < VSENSE < 1V SENSE BUFFER INTERNAL ADC HIGH REFERENCE 1μF REFH 2.2μF 0.1μF DIFF AMP 1μF REFL INTERNAL ADC LOW REFERENCE 222876 F09 Figure 9. Equivalent Reference Circuit 1.5V VCM 2.2μF 12k 0.75V 12k SENSE LTC2228/27/26 1μF 222876 F10 Figure 10. 1.5V Range ADC 222876fb 19 LTC2228/LTC2227/LTC2226 APPLICATIONS INFORMATION Input Range The input range can be set based on the application. The 2V input range will provide the best signal-to-noise performance while maintaining excellent SFDR. The 1V input range will have better SFDR performance, but the SNR will degrade by 3.8dB. See the Typical Performance Characteristics section. Driving the Clock Input The CLK input can be driven directly with a CMOS or TTL level signal. A sinusoidal clock can also be used along with a low jitter squaring circuit before the CLK pin (Figure 11). The noise performance of the LTC2228/LTC2227/LTC2226 can depend on the clock signal quality as much as on the analog input. Any noise present on the clock signal will result in additional aperture jitter that will be RMS summed with the inherent ADC aperture jitter. The nature of the received signals also has a large bearing on how much SNR degradation will be experienced. For high crest factor signals such as WCDMA or OFDM, where the nominal power level must be at least 6dB to 8dB below full scale, the use of these translators will have a lesser impact. The transformer in the example may be terminated with the appropriate termination for the signaling in use. The use of a transformer with a 1:4 impedance ratio may be desirable in cases where lower voltage differential signals are considered. The center tap may be bypassed to ground through a capacitor close to the ADC if the differential signals originate on a different plane. The use of a capacitor at the input may result in peaking, and depending on transmission line length may require a 10Ω to 20Ω series resistor to act as both a lowpass filter for high frequency noise that may be induced into the clock line by neighboring digital signals, as well as a damping mechanism for reflections. In applications where jitter is critical, such as when digitizing high input frequencies, use as large an amplitude as possible. Also, if the ADC is clocked with a sinusoidal signal, filter the CLK signal to reduce wideband noise and distortion products generated by the source. Figures 12 and 13 show alternatives for converting a differential clock to the single-ended CLK input. The use of a transformer provides no incremental contribution to phase noise. The LVDS or PECL to CMOS translators provide little degradation below 70MHz, but at 140MHz will degrade the SNR compared to the transformer solution. 4.7μF FERRITE BEAD 0.1μF CLK 100Ω LTC2238/ LTC2237/ LTC2236 223876 F12 IF LVDS USE FIN1002 OR FIN1018. FOR PECL, USE AZ1000ELT21 OR SIMILAR Figure 12. CLK Drive Using an LVDS or PECL-to-CMOS Converter CLEAN SUPPLY 4.7μF CLEAN SUPPLY FERRITE BEAD ETC1-1T 1k 5pF-30pF 0.1μF CLK SINUSOIDAL CLOCK INPUT 50Ω 1k CLK 0.1μF NC7SVU04 LTC2228/ LTC2227/ LTC2226 DIFFERENTIAL CLOCK INPUT 223876 F13 0.1μF 222876 F11 Figure 11. Single-Ended CLK Drive LTC2238/ LTC2237/ LTC2236 FERRITE BEAD VCM Figure 13. LVDS or PECL CLK Drive Using a Transformer 222876fb 20 LTC2228/LTC2227/LTC2226 APPLICATIONS INFORMATION Maximum and Minimum Conversion Rates DIGITAL OUTPUTS The maximum conversion rate for the LTC2228/LTC2227/ LTC2226 is 65Msps (LTC2228), 40Msps (LTC2227), and 25Msps (LTC2226). For the ADC to operate properly, the CLK signal should have a 50% (±5%) duty cycle. Each half cycle must have at least 7.3ns (LTC2228), 11.8ns (LTC2227), and 18.9ns (LTC2226) for the ADC internal circuitry to have enough settling time for proper operation. Table 1 shows the relationship between the analog input voltage, the digital data bits and the overflow bit. An optional clock duty cycle stabilizer circuit can be used if the input clock has a non 50% duty cycle. This circuit uses the rising edge of the CLK pin to sample the analog input. The falling edge of CLK is ignored and the internal falling edge is generated by a phase-locked loop. The input clock duty cycle can vary from 40% to 60% and the clock duty cycle stabilizer will maintain a constant 50% internal duty cycle. If the clock is turned off for a long period of time, the duty cycle stabilizer circuit will require a hundred clock cycles for the PLL to lock onto the input clock. To use the clock duty cycle stabilizer, the MODE pin should be connected to 1/3VDD or 2/3VDD using external resistors. The lower limit of the LTC2228/LTC2227/LTC2226 sample rate is determined by droop of the sample-and-hold circuits. The pipelined architecture of this ADC relies on storing analog signals on small-valued capacitors. Junction leakage will discharge the capacitors. The specified minimum operating frequency for the LTC2228/LTC2227/LTC2226 is 1Msps. Table 1. Output Codes vs Input Voltage AIN+ – AIN– (2V RANGE) OF D11-D0 (OFFSET BINARY) D11-D0 (2’s COMPLEMENT) >+1.000000V +0.999512V +0.999024V 1 0 0 1111 1111 1111 1111 1111 1111 1111 1111 1110 0111 1111 1111 0111 1111 1111 0111 1111 1110 +0.000488V 0.000000V –0.000488V –0.000976V 0 0 0 0 1000 0000 0001 1000 0000 0000 0111 1111 1111 0111 1111 1110 0000 0000 0001 0000 0000 0000 1111 1111 1111 1111 1111 1110 –0.999512V –1.000000V