DEMO MANUAL DC798B LTC2480 16-Bit Delta Sigma ADC with Easy Drive Input Current Cancellation Description Demonstration circuit 798B features the LTC2480, a 16-bit high performance analog-to-digital converter (ADC). The LTC2480 features 0.12LSB linearity, 0.16LSB full-scale accuracy, 5V offset, and 1.45VRMS noise. The input is fully differential, with input common mode rejection of 140dB. The LTC2480 is available in a 10-pin DFN package and has an easy to use SPI interface. DC798B is a member of Linear Technology`s QuikEvalTM family of demonstration boards. It is designed to allow easy evaluation of the LTC2480 and may be connected directly to the target application's analog signals while using the DC590 USB Serial Controller board and supplied software to measure performance. The exposed ground planes allow proper grounding to prototype circuitry. After evaluating with Linear Technology's software, the digital signals can be connected to the end application's processor/controller for development of the serial interface. Design files for this circuit board are available at http://www.linear.com/demo L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and QuikEval is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. Board Photo MONITOR REFERENCE OR EXTERNAL REFERENCE SOURCE (SEE HARDWARE SET-UP) RIBBON CABLE TO DC590 CONTROLLER DIFFERENTIAL INPUT Figure 1. Connection Diagram dc798bf 1 DEMO MANUAL DC798B Quick Start Procedure Connect DC798B to a DC590 USB Serial Controller using the supplied 14-conductor ribbon cable. Connect DC590 to host PC with a standard USB A/B cable. Run the evaluation software supplied with DC590 or downloaded from http://www.linear.com/software. The correct program will be loaded automatically. Click the COLLECT button to start reading the input voltage. Details on software features are documented in the control panel's help menu. Tools are available for logging data, changing reference voltage, changing the number of points in the strip chart and histogram, and changing the number of points averaged for the DVM display. Figure 2. Software Screenshot 2 dc798bf DEMO MANUAL DC798B Hardware Setup Connection To DC590 Serial Controller J1 is the power and digital interface connector. Connect to DC590 serial controller with supplied 14-conductor ribbon cable. Jumpers JP1: Select the source for REF+. Two 5V references are available to suit different applications--an LT1790A-5 suitable for most applications using the lower gain settings and an LT1236 that allows full noise performance at the highest gain settings. An external reference can also be applied to the REF+ turret post if JP1 is removed completely. Analog Connections Analog signal connections are made via the row of turret posts along the edge of the board. Also, when connecting the board to an existing circuit the exposed ground planes along the edges of the board may be used to form a solid connection between grounds. GND: Three ground turrets are connected directly to the internal ground planes. VCC: This is the supply for the ADC. Do not draw any power from this point. REF+: Connected to the LTC2480 reference pin. If one of the onboard references is being used, the reference voltage may be monitored from this point. An external reference may be connected to these terminals if JP1 is removed. IN+/IN- : These are the differential inputs to the LTC2480. f0: An external conversion clock may be applied to the f0 turret to modify the frequency rejection characteristics or data output rate of the LTC2480. This should be a square wave with a low level equal to ground and a high level equal to VCC. While up to a 2MHz clock can be used, performance may be compromised. Refer to the LTC2480 data sheet. This terminal has a 5k pull-down resistor to keep it from floating when no signal is applied. EXPERIMENTS Input Noise One of the characteristics of the LTC2480 is that the 600nV input noise floor is far below the quantization level of 38V when a 5V reference is used and the PGA is set to a gain of 1. This means that the output will be stable if the input noise level is significantly below 38V. In this sense, the LTC2480 is a true 17 effective bit part, whereas many 16-bit SAR converters have several LSBs of noise. Solder a short wire from the IN- turret post to the IN+ turret post. Noise should be below the quantization level of the LTC2480. This will result in a noise reading of zero on the control software. Increase the gain setting to 256 (1x speed mode.) This results in a 149nV LSB size, allowing a true measurement of the input noise. Two references are included on DC798B; an LT1790-5.0 and an LT1236-5. For applications using the lower gain settings, the LT1790 is adequate. However, in the highest gain settings the LT1790 can elevate the input noise by a factor of 2 or 3. The LT1236 is significantly quieter than the LT1790 and is a good choice for very high resolution applications. The LT1790 is a good choice for low power applications or where small size is required. Common Mode Rejection Tie the two inputs (still connected together) to ground through a short wire and note the indicated voltage. Tie the inputs to REF+; the difference should be less than 0.5V due to the 140dB minimum CMRR of the LTC2480. Bipolar Symmetry To demonstrate the symmetry of the ADCs transfer function, connect a stable, low noise, floating voltage source (with a voltage less than VREF /2) from IN+ to IN- and note the indicated voltage. Reverse the polarity; the indicated voltage will typically be within one LSB of the first reading multiplied by -1. dc798bf 3 DEMO MANUAL DC798B EXPERIMENTS One convenient voltage source for this experiment is a single alkaline battery. While a battery has fairly low noise, it is sensitive to temperature drift. It is best to use a large (D-size) battery that is insulated from air currents. A better source is a battery powered series reference such as the LT1790. This part is available with output voltages of 1.25V, 2.048V, 2.5V, 3V, 3.3V, 4.096V and 5V. to ground. Apply a 10Hz, 2V peak-to-peak sine wave to IN+ through a 1F capacitor. No DC bias is required because the 2M to 3M input impedance of the LTC2480 tends to self-bias the input to mid-reference (see data sheet Applications Information for details.) Input Normal Mode Rejection Next, slowly increase the frequency to 60Hz (or 50Hz or 55Hz, depending on the selected rejection frequency.) The noise should be almost undetectable in the graph. Note that the indicated noise in ppm may still be above that of the datasheet specification because the inputs are not connected to a DC source. The LTC2480's SINC4 digital filter is trimmed to reject 50Hz or 60Hz line noise when operated with the internal conversion clock. To measure input normal mode rejection, connect IN- to a 2.5V source such as an LT1790-2.5 reference or 1k - 1k divider from the onboard 5V reference Start taking data. The input noise will be quite large, and the graph of output vs. time should show large variations. Parts List ITEM QTY REFERENCE PART DESCRIPTION MANUFACTURER/PART NUMBER Required Circuit Components 1 3 C1, C6, C7 CAP., X7R, 0.1F, 16V, 20%, 0402 TDK, C1005X7R1C104M 2 2 C2, C3 CAP., X5R, 4.7F, 6.3V, 20%, 0603 TDK, C1608X5R0J475M 3 1 C4 CAP., X5R, 1F, 6.3V, 20%, 0402 TDK, C1005X5R0J105M 4 1 C5 CAP., X7R, 0.01F, 25V, 10%, 0402 AVX, 04023C103KAT1A 5 8 E1 to E8 TESTPOINT, TURRET, .064" MILL-MAX, 2308-2 6 1 JP1 JMP, 3-PIN, 1 ROW, .079CC COMM-CON, 2802S-03-G1 7 1 SHUNTS FOR JP1 PIN 1 & 2 SHUNT, .079" CENTER COMM-CON CCIJ2MM-138G 8 1 J1 HEADER, 2 x 7 PIN, 0.079CC MOLEX, 87331-1420 9 4 R1, R3, R4, R5 RES., CHIP 4.99k 1/16W, 1%, 0402 AAC, CR05-4991FM 10 1 R2 RES., CHIP 100, 1/16W, 5%, 0402 AAC, CR05-101JM 11 1 U1 I.C., LTC2480CDD, DFN10DD LINEAR TECH., LTC2480CDD 12 1 U2 I.C., LT1790ACS6-5, SOT23-6 LINEAR TECH., LT1790ACS6-5 13 1 U3 I.C., 24LC025, TSSOP8 MICROCHIP, 24LC025 14 1 U4 I.C., LT1236ACS8-5, SO8 LINEAR TECH., LT1236ACS8-5 4 dc798bf DEMO MANUAL DC798B Schematic Diagram dc798bf 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. 5 DEMO MANUAL DC798B 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 (c) 2004, Linear Technology Corporation 6 dc798bf Linear Technology Corporation LT 1013 * PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 FAX: (408) 434-0507 www.linear.com LINEAR TECHNOLOGY CORPORATION 2012