DEMO MANUAL DC1925A LTC2378-20/LTC2377-20/LTC2376-20 20-Bit,1Msps/500ksps/250ksps, Low Power, SAR ADCs with 104dB SNR DESCRIPTION The LTC®2378-20, LTC2377-20 and LTC2376-20 are 20‑bit, low power, low noise SAR ADCs with serial outputs that operate from a single 2.5V supply. The following text refers to the LTC2378-20 but applies to all parts in the family, the only difference being the maximum sample rate. The LTC2378-20 supports a ±5V fully differential input range with a 104dB SNR, consumes only 21mW and achieves ±2ppm INL max with no missing codes at 20 bits. The DC1925A demonstrates the DC and AC performance of the LTC2378-20 in conjunction with the DC590 QuikEval™ and DC890 PScope™ data collection boards. Use the DC590 to demonstrate DC performance such as peak-to-peak noise and DC linearity. Use the DC890 if precise sampling rates are required or to demonstrate AC performance such as SNR, THD, SINAD and SFDR. The demonstration circuit 1925A is intended to show recommended grounding, component placement and selection, routing and bypassing for this ADC. Design files for this circuit board are available at http://www.linear.com/demo or scan the QR code on the back of the board. L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and QuikEval and PScope are trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. BOARD PHOTO Figure 1. DC1925A Connection Diagram dc1925af 1 DEMO MANUAL DC1925A ASSEMBLY OPTIONS Table 1. DC1925A Assembly Options ASSEMBLY VERSION U1 PART NUMBER MAX CONVERSION RATE NUMBER OF BITS MAX CLK IN FREQUENCY DC1925A-A LTC2378CMS-20 1Msps 20 80MHz DC1925A-B LTC2377CMS-20 500ksps 20 40MHz DC1925A-C LTC2376CMS-20 250ksps 20 20MHz DC890 QUICK START PROCEDURE Check to make sure that all switches and jumpers are set as shown in the connection diagram of Figure 1. In particular make sure that VCCIO (JP3) is set to the 2.5V position. Operating the DC1925A with the DC890 while JP3 is in the 3.3V position will cause noticeable performance degradation in SNR and THD. The default connections configure the ADC to use the onboard reference and regulators to generate the required common mode voltages. The analog input is DC coupled. Connect the DC1925A to a DC890 USB high speed data collection board using connector P1. Then, connect the DC890 to a host PC with a standard USB A/B cable. Apply ±9V to the indicated terminals. Then apply a low jitter differential sine source to J2 and J4. Connect a low jitter 80MHz 2.5VP-P sine wave or square wave to connector J1. Note that J1 has a 50Ω termination resistor to ground. Run the PScope software (PScope.exe version K73 or later) supplied with the DC890 or download it from www. linear.com/software. Complete software documentation is available from the Help menu. Updates can be downloaded from the Tools menu. Check for updates periodically as new features may be added. The PScope software should recognize the DC1925A and configure itself automatically. Click the Collect button (See Figure 5) to begin acquiring data. The Collect button then changes to Pause, which can be clicked to stop data acquisition. DC590 SETUP IMPORTANT! To avoid damage to the DC1925A, make sure that VCCIO (JP6) of the DC590 is set to 3.3V before connecting the DC590 to the DC1925A. VCCIO (JP3) of the DC1925A should be in the 3.3V position for DC590 operation. To use the DC590 with the DC1925A, it is necessary to apply –9V and ground to the –9V and GND terminals. Connect the DC590 to a host PC with a standard USB A/B cable. Connect the DC1925A to a DC590 USB serial controller using the supplied 14-conductor ribbon cable. Apply a signal source to J4 and J2 or J4 depending on how the DC1925A is configured. Run the QuikEval software (version K92-01 or later) supplied with the DC590 or download it from www.linear. com/software. The correct control panel will be loaded automatically. Click the Collect button (See Figure 6) to begin reading the ADC. dc1925af 2 DEMO MANUAL DC1925A DC1925A SETUP DC Power Reference The DC1925A requires ±9VDC and draws approximately 100mA. Most of the supply current is consumed by the CPLD, op amps, regulators and discrete logic on the board. The +9VDC input voltage powers the ADC through LT1763 regulators which provide protection against accidental reverse bias. Additional regulators provide power for the CPLD and op amps. See Figure 1 for connection details. The default reference is a LTC6655 5V reference. If an external reference is used it must settle quickly in the presence of glitches on the REF pin. To use an external reference, unsolder R37 and apply the reference voltage to the VREF terminal. Clock Source You must provide a low jitter 2.5VP-P (if VCCIO is in the 3.3V position, the clock amplitude should be 3.3VP-P) sine or square wave to J1. The clock input is AC coupled so the DC level of the clock signal is not important. A clock generator like the Rohde & Schwarz SMB100A or the DC1216A-C is recommended. Even a good clock generator can start to produce noticeable jitter at low frequencies. Therefore it is recommended for lower sample rates to divide down a higher frequency clock to the desired input frequency. The ratio of clock frequency to conversion rate is 80:1. If the clock input is to be driven with logic, it is recommended that the 50Ω terminator (R5) be removed. Slow rising edges may compromise the SNR of the converter in the presence of high amplitude higher frequency input signals. Data Output Parallel data output from this board (0V to 2.5V default), if not connected to the DC890, can be acquired by a logic analyzer, and subsequently imported into a spreadsheet, or mathematical package depending on what form of digital signal processing is desired. Alternatively, the data can be fed directly into an application circuit. Use Pin 50 of P1 to latch the data. The data can be latched using either edge of this signal. The data output signal levels at P1 can also be changed to 0V to 3.3V if the application circuit requires a higher voltage. This is accomplished by moving VCCIO (JP3) to the 3.3V position. Analog Input The default driver for the analog inputs of the LTC2378-20 on the DC1925A is shown in Figure 2. This circuit buffers a fully differential 0V to 5V input signal applied at AIN+ and AIN–. In addition, this circuit bandlimits the input frequencies to approximately 1.2MHz. Alternatively, if your application circuit requires a singleended signal to drive the ADC, the circuit shown in Figure 3 can be used. The circuit of Figure 3 converts a singleended signal to the fully-differential signal required by the ADC. Additionally, this circuit further bandlimits the input frequencies to 100kHz which is the bandwidth limit of the ADC for low distortion performance. The single-ended-to-differential circuit results in reduced THD performance, (approximately –112dB) due to the slight phase shift of the inverting op amp. The circuit in Figure 3 can be implemented on the DC1925A by removing R44 and R52 and adding R57 and R58. At this point it will only be necessary to drive AIN+ (J4). Figure 2. Fully-Differential Driver dc1925af 3 DEMO MANUAL DC1925A DC1925A SETUP Figure 3. Single-Ended-to-Differential Driver Data Collection For SINAD, THD or SNR testing a low noise, low distortion differential output sine generator such as the Stanford Research DS360 should be used. A low jitter RF oscillator such as the Rohde & Schwarz SMB100A or DC1216A-C is used as the clock source. This demo board is tested in house by attempting to duplicate the FFT plot shown in the typical performance characteristics of the LTC2378-20 data sheet. This involves using an 80MHz clock source, along with a differential output sinusoidal generator at a frequency of 2.0kHz. The input signal level is approximately –1dBfs. The input is level shifted and filtered with the circuit shown in Figure 4. A typical FFT obtained with DC1925A is shown in Figure 5. Note that to calculate the real SNR, the signal level (F1 amplitude = –0.998dB) has to be added back to the SNR that PScope displays. With the example shown in Figure 5 this means that the actual SNR would be 103.668dB instead of the 102.67dB that PScope displays. Taking the RMS sum of the recalculated SNR and the THD yields a SINAD of 103.62dB which is fairly close to the typical number for this ADC. Figure 4. Differential Level Shifter There are a number of scenarios that can produce misleading results when evaluating an ADC. One that is common is feeding the converter with a frequency, that is a submultiple of the sample rate, and which will only exercise a small subset of the possible output codes. The proper method is to pick an M/N frequency for the input sine wave frequency. N is the number of samples in the FFT. M is a prime number between one and N/2. Multiply M/N by the sample rate to obtain the input sine wave frequency. Another scenario that can yield poor results is if you do not have a sine generator capable of ppm frequency accuracy or if it cannot be locked to the clock frequency. You can use an FFT with windowing to reduce the “leakage” or spreading of the fundamental, to get a close approximation of the ADC performance. If windowing is required, the Blackman-Harris 92dB window is recommended. If an amplifier or clock source with poor phase noise is used, windowing will not improve the SNR. Layout As with any high performance ADC, this part is sensitive to layout. The area immediately surrounding the ADC on the DC1925A should be used as a guideline for placement, and routing of the various components associated with the ADC. Here are some things to remember when laying out a board for the LTC2378-20. A ground plane is necessary to obtain maximum performance. Keep bypass capacitors as close to supply pins as possible. Use low impedance returns directly to the ground plane for each bypass capacitor. Use of a symmetrical layout around the analog inputs will minimize the effects of parasitic elements. Shield analog dc1925af 4 DEMO MANUAL DC1925A DC1925A SETUP input traces with ground to minimize coupling from other traces. Keep traces as short as possible. Component Selection When driving a low noise, low distortion ADC such as the LTC2378-20, component selection is important so as to not degrade performance. Resistors should have low values to minimize noise and distortion. Metal film resistors are recommended to reduce distortion caused by self heating. Because of their low voltage coefficients, to further reduce distortion NPO or silver mica capacitors should be used. Any buffer used to drive the LTC2378-20 should have low distortion, low noise and a fast settling time such as the LT6203. Figure 5. PScope Screen Shot dc1925af 5 DEMO MANUAL DC1925A DC1925A SETUP Figure 6. QuikEval Screen Shot dc1925af 6 DEMO MANUAL DC1925A DC1925A JUMPERS Definitions JP1 – EEPROM is for factory use only. Leave this in the default WP position. JP2 – V+ Selects 8V or 5V for V+. The default is position is 8V. Setting V+ to 5V is useful for evaluating single 5V supply operation of the buffer when operating the ADC with Digital Gain Compression turned on. JP3 – VCCIO sets the output levels at P1 to either 3.3V or 2.5V. Use 2.5V to interface to the DC890 which is the default setting. Use 3.3V to interface to the DC590. JP4 – VCM sets the DC bias for AIN+ and AIN– if the single ended to differential mode is enabled, and the inputs are AC coupled. VREF/2 is the default setting. JP5 – V– Selects –3.6V or ground for V–. The default setting is –3.6V. Setting V– to ground is useful for evaluating single supply operation of the buffer when operating the ADC with Digital Gain Compression turned on. JP6 – FS selects whether the Digital Gain Compression is on or off. In the VREF position, Digital Gain Compression is off and the analog input range at AIN+ and AIN– is 0V to VREF. In the 0.8VREF position, Digital Gain Compression is turned on and the analog input range at AIN+ and AIN– is 0.1VREF to 0.9VREF. The default setting is off. JP7 – Coupling selects AC or DC coupling of AIN+. The default setting is DC. JP8 – Coupling selects AC or DC coupling of AIN–. The default setting is DC. dc1925af 7 DEMO MANUAL DC1925A PARTS LIST ITEM QTY REFERENCE PART DESCRIPTION MANUFACTURER/PART NUMBER DC1925A General BOM 1 13 C1-C5, C7, C10, C11, C13-C16, C56 CAP., X7R, 0.1µF, 16V 10% 0603 AVX, 0603YC104KAT2A 2 10 C6, C9, C18, C24, C26, C29, C59, C60, C62, C69 CAP., X5R, 10µF, 6.3V 20% 0603 AVX, 06036D106MAT2A 3 1 C8 CAP., X7R, 1µF, 16V 10% 0603 AVX, 0603YC105KAT2A 4 9 C12, C19, C42, C43, C45, C47, C48, C57, C77 CAP., X7R, 0.1µF, 25V 20% 0603 AVX, 06033C104MAT2A 5 12 C17, C22, C25, C28, C40, C44, C49, C51, C54, C55, C58, C74 CAP., X5R, 1µF, 25V 10% 0603 X5R OK AVX, 06033D105KAT2A 6 1 C20 CAP., X7R, 47µF, 10V 10% 1210 MURATA, GRM32ER71A476KE15L 7 1 C21 CAP., X5R, 22µF, 25V 20% 1210 AVX, 12103D226MAT2A 8 4 C23, C27, C30, C50 CAP., X7R, 0.01µF, 25V 10% 0603 AVX, 06033C103KAT2A 9 8 C31, C32, C33, C34, C35, C36, C37, C38 CAP., X7R, 0.1µF, 16V 10% 0402 AVX, 0402YC104KAT2A 10 0 C39, C61, C63-C67, C70, C75, C76 CAP., OPT, 0603 OPTION 11 1 C46 CAP., X5R, 2.2µF, 10V 10% 0603 AVX, 0603ZD225KAT2A 12 2 C52 ,C53 CAP., X5R, 10µF, 25V 10% 0805 AVX, 08053D106KAT2A 13 1 C68 CAP., COG, 15pF, 50V 10% 0603 AVX, 06035A150KAT2A 14 2 C71, C73 CAP., NPO, 6800pF, 50V 5% 1206 MURATA, GRM3195C1H682JA01D 15 1 C72 CAP., NPO, 3300pF, 50V 10% 1206 AVX, 12065A332KAT2A 16 1 C78 CAP., X5R, 4.7µF, 6.3V 20% 0603 AVX, 06036D475MAT2A 17 7 E1, E2, E3, E4, E5, E9, E10 TEST POINT, TURRET, 0.061 MILL MAX, 2308-2-00-80-00-00-07-0 18 3 E6, E7, E8 TESTPOINT, TURRET, 0.094, PBF MILL MAX, 2501-2-00-80-00-00-07-0 19 8 JP1-JP8 HEADER, 3-PIN SINGLE ROW 0.100 SAMTEC, TSW-103-07-L-S 20 3 J1, J2, J4 CONNECTOR, BNC CONNEX, 112404 21 1 J3 CONN HEADER 14-POS 2MM VERT GOLD MOLEX, 87831-1420 22 1 J5 HEADER, 2X5, 0.100" SAMTEC, TSW-105-07-L-D 23 4 MH1, MH2, MH3, MH4 STANDOFF, NYLON 0.25" KEYSTONE, 8831 (SNAP ON) 24 4 R1, R3, R8, R15 RES., CHIP, 33Ω, 1/10W, 5% 0603 YAGEO, RC0603JR-0733RL 25 8 R2, R6, R7, R13, R19, R24, R29, R43 RES., CHIP, 1k, 1/10W, 1% 0603 YAGEO, RC0603JR-071KL 26 2 R4, R9 RES., CHIP, 0Ω, 1/16W, 0402 YAGEO, RC0402JR-070RL 27 1 R5 RES., CHIP, 49.9Ω, 1/4W, 1% 1206 YAGEO, RC1206FR-0749R9L 28 4 R10, R11, R12, R81 RES., CHIP, 4.99k, 1/10W, 1% 0603 YAGEO, RC0603FR-74K99L 29 24 R14, R61-R79, R82, R83, R84, R85 RES., CHIP, 33Ω, 1/16W, 5% 0402 YAGEO, RC0402JR-0733RL 30 1 R16 RES., CHIP, 300Ω, 1/16W, 5% 0402 YAGEO, RC0402JR-07300RL 31 1 R17 RES., CHIP, 2k, 1/10W, 5% 0603 YAGEO, RC0603JR-072KL 32 2 R18, R38 RES., CHIP, 249Ω, 1/10W, 1% 0603 YAGEO, RC0603FR-07249RL 33 3 R20, R22, R59 RES., CHIP, 1k, 1/16W, 5% 0402 YAGEO, RC0402JR-071KL 34 1 R21 RES., CHIP, 10k, 1/10W, 5% 0603 YAGEO, RC0603JR-0710KL 35 1 R23 RES., CHIP, 6.49k, 1/10W, 1% 0603 YAGEO, RC0603FR-076K49L 36 1 R25 RES., CHIP, 1.69k, 1/10W, 1% 0603 YAGEO, RC0603FR-071K69L 37 1 R26 RES., CHIP, 1.54k, 1/10W, 1% 0603 YAGEO, RC0603FR-071K54L dc1925af 8 DEMO MANUAL DC1925A PARTS LIST ITEM QTY REFERENCE PART DESCRIPTION MANUFACTURER/PART NUMBER 38 1 R27 RES., CHIP, 2.8k, 1/10W, 1% 0603 YAGEO, RC0603FR-072K8L 39 2 R28, R80 RES., CHIP, 2k, 1/10W, 1% 0603 YAGEO, RC0603FR-072KL 40 10 R30, R37, R39, R41, R44, R46, R50, R52, R53, R55 RES., CHIP, 0Ω, 1/10W, 0603 YAGEO, RC0603JR-070RL 41 4 R33, R34, R86, R87 RES., CHIP, 499Ω, 1/10W, 1% 0603 VISHAY, CRCW0603499RFKEA 42 10 R35, R36, R40, R45, R47, R49, R54, R56, R57, R58 RES., CHIP, OPT, 0603 OPTION 43 1 R42 RES., CHIP, 5.62k, 1/10W, 1% 0603 YAGEO, RC0603FR-075K62L 44 2 R48,R51 RES., CHIP, 10Ω, 1/10W, 1% 0603 YAGEO, RC0603FR-0710RL 45 1 R60 RES., CHIP, 10k, 1/16W, 5% 0402 YAGEO, RC0402JR-0710KL 46 1 R88 RES., CHIP, 1Ω, 1/10W, 5% 0603 YAGEO, RC0603JR-071RL 47 2 U2, U4 IC, UNBUFFERED INVERTER, SC70-5 FAIRCHILD, NC7SVU04P5X 48 1 U3 IC, D FLIP-FLOP, US8 ON SEMI., NL17SZ74USG 49 1 U5 IC, OP AMP, LOW NOISE, TSOT-23 LINEAR TECH., LT6202CS5#PBF 50 1 U6 IC, SINGLE SPST BUS SWITCH, SC70-5 FAIRCHILD, NC7SZ66P5X 51 1 U7 IC, SERIAL EEPROM, TSSOP MICROCHIP, 24LC024-I/ST 52 2 U8, U9 IC, UHS INVERTER, SC70-5 FAIRCHILD, NC7SZ04P5X 53 2 U10, U18 IC, DUAL OP AMP, MS8 LINEAR TECH., LT6203CMS8#PBF 54 1 U11 IC, MAX II CPLD, TQFP100 ALTERA, EPM240GT100C5N 55 1 U12 IC, MICROPOWER REGULATOR, SO-8 LINEAR TECH., LT1763CS8-1.8#PBF 56 2 U13, U16 IC, MICROPOWER REGULATOR, SO-8 LINEAR TECH., LT1763CS8#PBF 57 1 U14 IC, MICROPOWER REGULATOR, SO-8 LINEAR TECH., LT1763CS8-2.5#PBF 58 1 U15 IC, VOLTAGE REFERENCE, MSOP LINEAR TECH., LTC6655BHMS8-5#PBF 59 1 U17 IC, MICROPOWER NEG. REGULATOR LINEAR TECH., LT1964ES5-SD#PBF 60 8 XJP1-XJP8 SHUNT, 0.100 CENTERS SAMTEC, SNT-100-BK-G 61 2 STENCIL, (TOP & BOTTOM) STENCIL DC1925A 1 1 GENERAL BOM DC1925A 2 1 IC, HIGH SPEED, LOW NOISE SAR ADC LINEAR TECH., LTC2378CMS-20#PBF 3 1 FAB, PRINTED CIRCUIT BOARD DEMO CIRCUIT 1925A-2 1 1 GENERAL BOM DC1925A 2 1 IC, HIGH SPEED, LOW NOISE SAR ADC LINEAR TECH., LTC2377CMS-20#PBF 3 1 FAB, PRINTED CIRCUIT BOARD DEMO CIRCUIT 1925A-2 1 1 GENERAL BOM DC1925A 2 1 IC, HIGH SPEED, LOW NOISE SAR ADC LINEAR TECH., LTC2376CMS-20#PBF 3 1 FAB, PRINTED CIRCUIT BOARD DEMO CIRCUIT 1925A-2 DC1925A-A U1 DC1925A-B U1 DC1925A-C U1 dc1925af 9 DEMO MANUAL DC1925A SCHEMATIC DIAGRAM dc1925af 10 DEMO MANUAL DC1925A SCHEMATIC DIAGRAM dc1925af Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 11 DEMO MANUAL DC1925A DEMONSTRATION BOARD IMPORTANT NOTICE Linear Technology Corporation (LTC) provides the enclosed product(s) under the following AS IS conditions: This demonstration board (DEMO BOARD) kit being sold or provided by Linear Technology is intended for use for ENGINEERING DEVELOPMENT OR EVALUATION PURPOSES ONLY and is not provided by LTC for commercial use. As such, the DEMO BOARD herein may not be complete in terms of required design-, marketing-, and/or manufacturing-related protective considerations, including but not limited to product safety measures typically found in finished commercial goods. As a prototype, this product does not fall within the scope of the European Union directive on electromagnetic compatibility and therefore may or may not meet the technical requirements of the directive, or other regulations. If this evaluation kit does not meet the specifications recited in the DEMO BOARD manual the kit may be returned within 30 days from the date of delivery for a full refund. THE FOREGOING WARRANTY IS THE EXCLUSIVE WARRANTY MADE BY THE SELLER TO BUYER AND IS IN LIEU OF ALL OTHER WARRANTIES, EXPRESSED, IMPLIED, OR STATUTORY, INCLUDING ANY WARRANTY OF MERCHANTABILITY OR FITNESS FOR ANY PARTICULAR PURPOSE. EXCEPT TO THE EXTENT OF THIS INDEMNITY, NEITHER PARTY SHALL BE LIABLE TO THE OTHER FOR ANY INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES. The user assumes all responsibility and liability for proper and safe handling of the goods. Further, the user releases LTC from all claims arising from the handling or use of the goods. Due to the open construction of the product, it is the user’s responsibility to take any and all appropriate precautions with regard to electrostatic discharge. Also be aware that the products herein may not be regulatory compliant or agency certified (FCC, UL, CE, etc.). No License is granted under any patent right or other intellectual property whatsoever. LTC assumes no liability for applications assistance, customer product design, software performance, or infringement of patents or any other intellectual property rights of any kind. LTC currently services a variety of customers for products around the world, and therefore this transaction is not exclusive. Please read the DEMO BOARD manual prior to handling the product. Persons handling this product must have electronics training and observe good laboratory practice standards. Common sense is encouraged. This notice contains important safety information about temperatures and voltages. For further safety concerns, please contact a LTC application engineer. Mailing Address: Linear Technology 1630 McCarthy Blvd. Milpitas, CA 95035 Copyright © 2004, Linear Technology Corporation dc1925af 12 Linear Technology Corporation LT 0413 • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com LINEAR TECHNOLOGY CORPORATION 2013