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1300 Henley Court Pullman, WA 99163 509.334.6306 www.digilentinc.com This manual applies to Rev. B; Revised October 10, 2013 Written by Mircea Dabacan, PhD, Technical University of Cluj-Napoca Romania Overview The Digilent Analog Discovery™, developed in conjunction with Analog Devices Inc., is a multifunction instrument that can measure, record and generate analog and digital signals. The small, portable and low-cost Analog Discovery (Figure 1) was created so that engineering students could work with analog and digital circuits anytime, anywhere - right from their PC. The Analog Discovery’s analog and digital inputs and outputs connect to a circuit using simple wire probes. Inputs and outputs are controlled using the free PCbased Waveforms software that can configure the Discovery to work as any one of several traditional instruments. Instruments include: Figure 1. Analog Discovery used in a circuit design experiment             Two channel oscilloscope (1MΩ, ±25V, differential, 14 bit, 100Msample/sec, 5MHz bandwidth); Two channel arbitrary function generator (22Ω, ±5V, 14 bit, 100Msample/sec, 5MHz bandwidth); Stereo audio amplifier to drive external headphones or speakers with replicated AWG signals; 16-channel digital logic analyzer (3.3V CMOS, 100Msample/sec)*; 16-channel pattern generator (3.3V CMOS, 100Msample/sec)*; 16-channel virtual digital I/O including buttons, switches and LEDs –good for logic trainer applications*; Two input/output digital trigger signals for linking multiple instruments (3.3V CMOS); Two power supplies (+5V at 50mA, -5V at 50mA). Single channel voltmeter (AC, DC, ±25V); Network analyzer – Bode, Nyquist, Nichols transfer diagrams of a circuit. Range: 1Hz to 10MHz; Spectrum Analyzer - power spectrum and spectral measurements (noise floor, SFDR, SNR, THD, etc.); Digital Bus Analyzers (SPI, I2C, UART, Parallel); The Analog Discovery was designed for students in typical university-based circuits and electronics classes. Its features and specifications, including operating from USB power, a small and portable form factor, and the ability to be used by students in a variety of environments at low cost, are based directly on inputs from many professors at many universities. Meeting all the requirements proved challenging, and resulted in some new and innovative circuits. This document is a reference for the Analog Discovery’s electrical functions and operations. This reference also provides a description of the hardware’s features and limitations. It is not intended to provide enough information to enable complete duplication of the Analog Discovery, or to allow users to design custom configurations for programmable parts in the design. DOC#:502-244 Copyright Digilent, Inc. All rights reserved. Other product and company names mentioned may be trademarks of their respective owners. Page 1 of 38 Analog Discovery Technical Reference Manual Architectural Overview and Block Diagram Analog Discovery’s high-level block diagram is presented in figure 2 below. The core of the Analog Discovery is the Xilinx Spartan 6 FPGA (specifically, the XC6SLX16-1L device). The Waveforms software automatically programs Discovery’s FPGA at start-up with a configuration file designed to implement a multi-function test and measurement instrument. Once programmed, the FPGA communicates with the PC-based Waveforms software via a USB2.0 connection. The Waveforms software works with the FPGA to control all the functional blocks of the Analog Discovery, including setting parameters, acquiring data, and transferring and storing data. Signals in the Analog Input block, also called the Scope, use “SC” indexes to indicate they are related to the scope block. Signals in the Analog Output block, also called AWG, use “AWG” indexes, and signals in the Digital block use a D index – all the instruments offered by Discovery and Waveforms use the circuits in these three blocks. Signal and equations also use certain naming conventions. Analog voltages are prefixed with a “V” (for Voltage), and suffixes and indexes are used in various ways: to specify the location in the signal path (IN, MUX, BUF, ADC, etc.); to indicate the related instrument (SC, AWG, etc.); to indicate the channel (1 or 2); and to indicate the type of signal (P, N, or diff). Referring to the block diagram below,  The Analog Inputs/Scope instrument block includes: - Input Divider and Gain Control – high bandwidth input adapter/divider. High or Low Gain can be selected by the FPGA; - Buffer – high impedance buffer; - Driver – provides appropriate signal levels and protection to the ADC. Offset voltage is added for vertical position setting; - Scope Reference and Offset – generates and buffers reference and offset voltages for the scope stages; - ADC – the Analog to Digital Converter for both scope channels.  The Arbitrary Outputs/AWG instrument block includes: - DAC – the Digital to Analog Converter for both AWG channels; - I/V – current to bipolar voltage converters; - Out – output stages; - Audio – audio amplifiers for headphone.  A precision Oscillator and a Clock Generator provide a high quality clock signal for the AD and DA converters.  The Digital I/O block exposes protected access to the FPGA pins assigned for the Digital Pattern Generator and Logic Analyzer.  The Power Supplies and Control block generates all internal supply voltages and user supply voltages. The control block also monitors the device power consumption for USB compliance (all power for the Analog Discovery is supplied via the USB connection). Under the FPGA control, power for unused functional blocks can be turned off.  The USB controller interfaces with the PC for programming the volatile FPGA memory after power on or when a new configuration is requested. After that, it performs the data transfer between the PC and FPGA.  The Calibration Memory stores all calibration parameters. Except for the “Probe Calibration” trimmers in the scope Input divider, the Analog Discovery includes no analog calibration circuitry. Instead, a calibration operation is performed at manufacturing (or by the user), and parameters are stored in memory. The WaveForms software uses these parameters to correct the acquired data and the generated signals. Copyright Digilent, Inc. All rights reserved. Other product and company names mentioned may be trademarks of their respective owners. Page 2 of 38 Analog Discovery Technical Reference Manual In the sections that follow, schematics are not shown separately for identical blocks. For example, the Scope Input Divider and Gain Selection schematic is only shown for channel 1 since the schematic for channel 2 is identical. Indexes are omitted where not relevant. As examples, in equation ( 4 ) below, does not contain the instrument index (which by context is understood to be the Scope), nor the channel index (because the equation applies to both channels 1 and 2). In equation ( 3 ), the type index is also missing, because and refer to any of P (positive), N (negative) or diff (differential) values. Analog Inputs (Scope) SC1 Osc Buffer Input divider and gain selection Driver I/V Audio Out Scope reference and offset SC2 Analog Outputs (AWG) Clock generator ADC DAC AWG1 AWG ref and offset Out Input divider and gain selection Buffer Driver Left I/V AWG2 Audio Right FPGA PC USB Prog Data USB ctrl Osc Internal power Calibration Memory Power Supplies and Control Digital I/O D0…D15 T0…T1 V+, V- Figure 2. Analog Discovery Hardware Block Diagram Copyright Digilent, Inc. All rights reserved. Other product and company names mentioned may be trademarks of their respective owners. Page 3 of 38 Analog Discovery Technical Reference Manual Scope Important Note: Unlike traditional inexpensive scopes, the Analog Discovery inputs are fully differential. However, a GND connection to the circuit under test is needed to provide a stable common mode voltage. The Analog Discovery GND reference is connected to the USB GND. Depending on the PC powering scheme, and other PC connections (Ethernet, audio, etc. – which might be also grounded) the Analog Discovery GND reference might be connected to the whole system GND and ultimately to the Power Network protection (EARTH ground). The circuit under test might be also connected to EARTH or might be floating. For safety reasons, it is the user’s responsibility to understand the powering and grounding scheme and make sure that there is a common GND reference between the Analog Discovery and the circuit under test, and that the common mode and differential voltages do not exceed the limits shown in equation ( 1 ). Furthermore, for distortion-free measurements, the common mode and differential voltages need to fit into the linear range shown in Figures 10 and 11. For those applications which scope GND cannot be the USB ground, a USB isolation solution, such as what is described in ADI’s CN-0160 can be used, however, this will limit things to USB full speed (12 Mbps), and will impact the update rate (screen refresh rates, not sample rates) of the Analog Discovery. Scope Input Divider and Gain Selection Figure 3 shows the scope input divider and gain selection stage. Two symmetrical R-C dividers provide: - Scope input impedance = 1MOhm || 24pF - Two different attenuations for High Gain/Low Gain (10:1) - Controlled capacitance, much higher than the parasitical capacitance of subsequent stages - Constant attenuation and high CMMR over a large frequency range (trimmer adjusted) - Protection for overvoltage (with the ESD diodes of the ADG612 inputs) The maximum voltage rating for scope inputs is limited by C1 thru C24 to: (1) The maximum swing of the input signal to avoid signal distortion by opening the ADG612 ESD diodes is (for both Low Gain and High Gain): (2) An analog switch (ADG612) allows selecting High Gain versus Low Gain (EN_HG_SC1, EN_LG_SC1) signals from the FPGA. The P and N branches of the differential path are switched together. The ADG612 quad switch was used because it provides excellent impedance and bandwidth parameters: - 1 pC charge injection - ±2.7 V to ±5.5 V dual-supply operation - 100 pA maximum at 25°C leakage currents - 85 Ω on resistance - Rail-to-rail switching operation Copyright Digilent, Inc. All rights reserved. Other product and company names mentioned may be trademarks of their respective owners. Page 4 of 38 Analog Discovery Technical Reference Manual - Typical power consumption: