Floating multi-channel picoammeter

Transcription

1 Floating multi-channel picoammeter University of Zagreb A Utrobičić March 19, 2015

2 Current measurements requirements floating current measurement at high potential > 5kV current resolution < 100 pa small dimensions for easier fitting to chamber low leakage current to ground ultra low power requirements for longer time measurement, when used on the battery source single side supply digital readout 10 channels (one current measurement for each GEM electrode + drift cathode) simultaneous high speed, real time readout for all channels University of Zagreb () Floating multi-channel picoammeter March 19, / 17

3 CYPRESS CY7C68Y85 Choice of the concept Multiple current measurement nodes send current data to the central FPGA board which is used as a data router to the PC by USB I in I V MSPs5YFlY85 I out VE Vk ZeroXburden pammeterxboard 86Xbit ΣXΔ ADC μp core U S I VRXLINK POF TX 5y7XVXLiPo PLASTICXOPTICAL FIBER 5YYXkbps VRXLINK POF RXX8 VRXLINK POF RXXl VRXLINK POF RXX5 RXX8X MODULE RXXlX MODULE RXX5X MODULE PACKETX GENERATOR MODULE SAMPLE GENERATOR 5lXkbyte FIFO FXlLP USB INTF USB I in I V MSPs5YFlY85 I out VE Vk ZeroXburden pammeterxboard 86Xbit ΣXΔ ADC μp core U S I VRXLINK POF TX 5y7XVXLiPo PLASTICXOPTICAL FIBER 5YYXkbps VRXLINK POF RXXN RXXNX MODULE ALTERAXCycloneXIII FPGAXUSBXBOARD University of Zagreb () Floating multi-channel picoammeter March 19, / 17

4 Choice of the low power components for the current measurement node Σ -ADC was chosen to fulfil resolution requirements LMC6442 (4 µw, I b =5 fa typ) was used for current signal conditioning MSP430F2013 with integrated 16-bit Σ -ADC and advanced low-power modes capabilities was used as a heart of the measurement node Avago Versatile-Link 5 Mbps optical fibre transmitter is employed to output digitized data Entire module is supplied from a single 37 V battery cell (Li-ion or LiPo) I in I out I V ZeroXburden V+ V- pammeterxboard Bits Slope MSP430F Xbit ΣXΔ ADC Sigma-Delta μp core U S I SAR k 10k 100k 1M Samples per Second VRXLINK POF TX 37XVXLiPo University of Zagreb () Floating multi-channel picoammeter March 19, / 17

5 Analog signal conditioning section Zero voltage burden OP-amp circuit was chosen to convert current to voltage Due to the zero voltage on the protection devices (D 1 and D 2 ), excellent linearity can be achieved when compared to the shunt measurement Common mode voltage is set to 12 V, which enables bidirectional current measurement using differential ADC input I IN I OUT 12 V REF R 1 R 4 V COM C 4 C 5 +33V D 1 D 2 L 1 IC 1A R 2 R 3 C 3 IC 1B C 1 C 2 V OUT V COM C 6 R 5 + ADC DIFF - INPUT University of Zagreb () Floating multi-channel picoammeter March 19, / 17

6 Serial data communication trough optical link A5 A4 A3 A2 A1 A0 D7 D6 D5 D4 D3 D2 D1 D0 0 0 { { START BIT ADDRESS DATA BYTE STOP BIT Data from the nodes is sent trough an optical fibre using USI port which is available on MSP430F2013 µc 14-bit asynchronous serial communication was emulated by a 16-bit serial stream, start and stop bit were added By dividing the 14-bit of useful data to 6-bit address and 8-bit data part it is possible to send multiple data from the node Only the current measurement is implemented for now and it is using 2 addresses Temperature and µc supply voltage can be measured and sent over the optical link if required University of Zagreb () Floating multi-channel picoammeter March 19, / 17

7 Packet #1 Packet #2 For the purpose of the current measurement 2 words are sent out from the measurement node to create one 16-bit current value Data rate is set to 500 kbps Sampling frequency is set to approx 1 khz The sampling time is crucial for the power consumption as it dictates the TX LED duty-cycle Maximum LED duty cycle for this settings (2-words, 500 kbps and 1 khz sampling time) is 64 % Hardware is capable to work with higher current sampling rate (5 khz max) if required However this would imply higher power consumption University of Zagreb () Floating multi-channel picoammeter March 19, / 17

8 FPGA code block diagram The entire FPGA code is written in Verilog HDL, schematics shown: Date: November 07, 2014 spi_parallelizer Project: nammeter packetizer:pack_inst CLKin FLAGD (GND) FLAGC (GND) FLAGA (GND) serial_in_4 serial_in_1 SLWR PKTEND SLRD SLOE FIFOADR0 FIFOADR1 CLK24 FD[150] serial_in_2 LD0 LD2 LD1 serial_in_5 serial_in_6 serial_in_7 serial_in_3 serial_in_8 FLAGB LD3 University of Zagreb () Floating multi-channel picoammeter March 19, / 17

9 Packetization of data from multiple channels START WORD CH0_VALUE CH1_VALUE CHN_VALUE XOR CHECK SUM Data obtained from multiple current sensors is inserted in the packet Packet consists of 16-bit start word, N 16-bit data words and 16 bit XOR checksum 16-bit data words represent current values for each of the current measurement channels As data is received asynchronously at 1 khz frequency the readout should be at least twice faster Packets with data from all channels are sent to FIFO at 2 khz frequency To obtain seamless streaming of data to PC over USB, 16 kword FIFO is implemented in the FPGA University of Zagreb () Floating multi-channel picoammeter March 19, / 17

10 Communication to FPGA trough USB using LabVIEW Cypress FX2LP integrated USB phy (CY7C60813) is used for communication to host PC FX2LP is operating in slave FIFO mode with only bulk IN endpoint enabled Additional FPGA FIFO enables proper communication when host CPU is highly loaded Windows driver is based on National Instruments VISA USB RAW bulk transfer, this enables easy integration to the LabVIEW application Transfer speed that can be achieved between FPGA and PC host is circa 20 Mbyte/s on USB20 port University of Zagreb () Floating multi-channel picoammeter March 19, / 17

11 Host application LabVIEW library for communication to the measurements system was developed Library consists of functions FIND, OPEN, READ and CLOSE and it can be easily integrated in any LabVIEW project It requires NI VISA to be installed Application is organized in two tabs: Measurement and Settings HIDE/SHOWdchannel USBdsamplingd frequency Notchdfilterdband,dHz Notchdfrequency,dHz Lowdpassdfilterd crossoverdfrequency EnabledLow-pass/ Notchdfilter STOPdthedapplication Calibration)data)for)each) picoammeter)the)parameter a)and)b are)marked)on)back of)the)each)of)the) picoammeter I=aH0ADC_VALUE2)z)b Number)of)data)points)in) each)transfer)defines) speed)of)the)execution)of the)main)loop)eg)1000 data)points)at)2)khz sampling)frequency)sets refresh)rate)to)2)hz Meandvaluesdofdthedone transferddatadblock StddDeviationsdofdthe onedtransferddatadblock University of Zagreb () Floating multi-channel picoammeter March 19, / 17

12 Prototype Ten prototypes are currently populated on a 2-layer PCB with dimensions mm Floating shield tied to the battery negative potential mitigates EMI radiated from the digital circuitry PCB + LiPo (37 V, 350 mah) battery are placed inside plastic housing with dimensions mm Plastic case has small pocket where NdFeB magnet is located for purpose of turning on/off picoammeter 30 mm 62 mm When battery fully charged it should last approx 9 days of continuous operation There are two pins left of the optical transmitter for connecting battery charger when battery is depleted University of Zagreb () Floating multi-channel picoammeter March 19, / 17

13 Low-cost FPGA board with ALTERA Cyclone III, is used to implement multi-channel receiving and buffering Simple optical RX daughter board for the main FPGA-USB board with 16 RX channels has been designed Optical receivers are populated on FPGA daughter board Board has 16 red LED diodes which indicates which channel is active University of Zagreb () Floating multi-channel picoammeter March 19, / 17

14 First measurements PicoAmmeter was connected to the Agilent 33500B waveform generator Current was set to mean value of 05 na by adjusting the voltage on the protection resistors R1 + R2 and recorded for 15 min Histogram of the RAW signal from the ADC was fitted to the gaussian Normalized histogram University of Zagreb ADC counts, 1 LSB = 4 pa () Floating multi-channel picoammeter March 19, / 17

15 Calibration procedure SHIELD Agilent 33500B PAD1 PMFapA PAD2 I cal 10MΩ Keithley 6485 V cal Keithley 6485 picoammeter was connected in series with our picoammeter and 10 MΩ resistor The resistor is necessary to convert voltage from Agilent 33500B waveform generator to current Special care must be taken when connecting picoammeters in series, high impedance terminals of the picoammeters must be connected to each other Low impedance terminals are connected to ground (Keithley) and calibration voltage (PMF pa) ADC data is fitted to the Keithley 6485 measurement using LSQ min fit University of Zagreb () Floating multi-channel picoammeter March 19, / 17

16 Specification sheet of the prototype Measuring range: from -125 na to 125 na It can be changed to lower by setting the internal PGA to other that 1x or by modifying R 3 Resolution (LSB): 4 pa at measuring range above FWHM: 6 pa, RAW signal, 1 khz sampling, 15 minutes Sampling frequency: 1 khz Current consuption: 15 ma at 1 khz sampling freq Supply voltage: 33 V-55 V - single lithium cell Optical connector: VR - Versatile Link (Avago) PCB dimensions: 54x235x9 mm Isolation voltage: N/A, defined by casing and length of fibre Battery: LiPo 37 V, 350 mah University of Zagreb () Floating multi-channel picoammeter March 19, / 17

17 Future work Implement autoranging feature by means of PGA (Programmable Gain Amplifier) that is already inbuilt inside the ADC of the µc We have sent prototypes to other groups for the testing purposes and we are waiting for their feedback so we can improve the device To make noise free DC/DC converter that will replace the battery supply and ensure HV isolation As we are depending on the third party FPGA board, we are considering do design our custom FPGA board with optical receivers and RAM We are considering usage of different interfaces to host PC than USB, ethernet? University of Zagreb () Floating multi-channel picoammeter March 19, / 17