Pulse Source and Measure Fundamentals

Transcription

1 Pulse Source and Measure Fundamentals May 2015

2 Goals & Overview Goal: To understand the critical differences between an SMU and an PMU Overview: Review of SMU Theory of Operation Comparison of PMU Theory of Operation with SMU Theory of Operation Summary of Differences

3 Background Need to understand the differences between an SMU and the 4225-PMU To do this we need to understand how a SMU works 3 3

4 SMU Theory Of Operation (1) A SMU provides simultaneous precision sourcing and precision measurement of voltage and current. The design of an SMU is based upon a classic control system with set points, feedback, and plant. The set point is the desired output level and limits. The feedback is the real-time sensing of a signal. The plant is the actual signal output at the DUT. 4 4

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6 SMU Theory of Operation (2): Operating Modes 4 Modes of Operation Meas I Meas I 1 3 Meas V Meas V Source I Source V

7 SMU Theory of Operation (3): Driven Guard SMU s generally have a driven guard A guard is a x1 buffer that makes sure that the guard terminal is at the same potential as the output terminal This is done to eliminate leakage paths and makes for quieter measurements. 7

8 SMU Theory of Operation (4): Current Source Mode In current source mode, the SMU is a high impedance current source with V-measure, I- measure, and V-limit capability 8

9 SMU Theory of Operation (5): Voltage Source Mode When acting as a voltage source, the SMU is a low impedance voltage source with I-measure, I- limit, and V-measure capability. 9 9

10 SMU Theory of Operation (6): 2-Wire Mode vs 4-Wire Mode An SMU can operate in 2-wire or 4-wire mode In 2-wire mode, it makes its measurement at the instrument terminals In 2-wire mode the SMU does not account for the drops due to the interconnect In 4-wire mode an additional connection is made at the DUT In 4-wire mode drops due to the cabling are eliminated 10 10

11 SMU Theory of Operation (7): Auto-Ranging When an SMU auto-ranges an approximate feedback loop control system is used. The approximate feedback loop holds the output signal constant while the SMU determines if it needs to change range

12 SMU Theory of Operation (8): Limit/Compliance The same approximate feedback control system is used during compliance determination When the SMU determines that it has hit a programmed limit it changes modes to maintain that limit If in voltage limit, the SMU switches to voltage source mode and sources the limit value If in current limit, the SMU switches to current source mode and sources the limit value 12

13 SMU Summary A SMU: Priority is accurate/precision sourcing and measurements. Timing is not as critical! Has 4 modes of operation Has programmable limits that occur in the analog domain Provides a guard signal Can operate in 2-wire (local sense) or 4-wire (remote sense) mode 13

14 K E I T H L E Y C O N F I D E N T I A L P R O P R I E T A R Y PMU Theory of Operation (1) A PMU can also source voltage The PMU is a pulser that has been modified to measure A PMU is time-oriented A PMU has only one real mode: Source V and Measure I and Measure V 14

15 The PMU is a VOLTAGE SOURCE! Pulse Section Measure Section 15

16 PMU Theory of Operation (3): 50 Ohm Internal Impedance PMU impedance is 50 ohms 50 ohms was chosen to match the Impedance of the coax transmission line which in turn reduces signal Vdut Reflections = Vint The PMU will set it s internal voltage to account for the 50 ohm impedance and the user s desired voltage The 50ohm internal impedance reduces the voltage available to the DUT The PMU does not have remote sense and cannot adjust for the 50 ohm drop in the analog domain To account for the drop, the PMU must first make a measurement and then make an adjustment Rdut (Rdut + Rint) 16

17 PMU Theory of Operation (4): Output Compensation Two ways for a PMU to adjust it s Output to account for losses: Output a pulse take a measurement evaluate the measurement adjust. This is called Load Line Effect Compensation (LLEC) Ask the user to tell it the approximate DUT impedance Calculate the appropriate internal voltage using a voltage divider equation. This is VERY different from a SMU in that a signal is first output, a measurement is taken, and adjustment is made. While the adjustments are being made, there is no signal being presented to the DUT. During this process, the DUT still only sees pulses of the programmed duration. Output Pulse Burst Get Good Measurement (ranging, etc) At Voltage? Y Put Data in Sheet, go to next sweep point N LLEC Looping Calculate new pulse level 17

18 PMU Theory of Operation (5): Auto-Ranging When a PMU autoranges, it follows a similar procedure to LLEC. That is, while the output is off, it evaluates the previous measurement to determine if it needs to change range. This is done because the PMU must give priority to timing. That is, it can only output a signal for as long as required by the user s timing parameters! This can cause some problems if the user sets an inappropriately short pulse width it may cause the PMU to take a reading on unsettled signal. And if the measurement is taken on an unsettled signal, the PMU may choose the wrong range during auto-ranging. Remember: A SMU has a source--range delay--user delay--measure sequence. That is, since it is a measurement priority instrument, readings are taken on (mostly) settled signals. 18

19 PMU Theory of Operation (6): Thresholds A PMU does not have compliance, it has thresholds Since a PMU is a time-priority instrument, The only way for it to determine if it has hit (or exceeded) a programmed threshold is after a pulse-measure burst is complete. This determination happens in the digital domain. Once it has determined that it has hit a threshold, it stops outputting pulses. The implications of this method is that the PMU can momentarily exceed the programmed threshold value because it will not know that it hit a threshold until after the pulse-measure sequence is complete! For a single pulse without LLEC it doesn t matter. When averaging, it could matter because there may be multiple pulses output before it can figure out that a threshold has been hit! 19

20 Other PMU / SMU Differences When a PMU outputs a DC signal, it is simply outputting a pulse whose amplitude and base are the same voltage. In fact, when in DC mode, the PMU will still use the timing parameters set in the timing dialog (unlike the SMU). Being time-oriented, a PMU can make measurements during very short pulses (10 s of ns). An SMU cannot its best pulse-measure time is around 100us. On the 4200, there are no KXCI commands for the PMU like there are for the SMU. 20

21 Summary PMU SMU Is a voltage source that simultaneously measures V and I Is a time accuracy priority instrument Output is off during auto-range range Determination Has thresholds and not compliance; cannot determine threshold state during measurement Has accurate timing Does not have remote sense Does not have guarding Does not have KXCI commands Can output and measure during short pulses (<< 1us) Is a 4 Mode Instrument Is a measurement accuracy/precision priority instrument Output is held on during auto-range range determination Can determine compliance status during measurement and can hold output to the compliance value Timing is limited by built in settling delays Has remote sense capability Has guarding Has KXCI commands Relatively slow pulse output with measure (fastest is around 100us)