femtoammeter design: development module for charged ... · cables and connectors shielding is a...
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GUSTAVO CASTRO
SYSTEM APPLICATIONS ENGINEER
PRECISION INSTRUMENTATION
Femtoammeter Design:
Development Module for
Charged Particle
Detection
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Femtoammeter Module
► Development module for sensors with low-level current output Direct interface to photodiodes, faraday
cups through SMA connector
► Features <10fA sensitivity with 10GW
transimpedance 400pA measurement range
Shielding
Isolation with ADuM3151
Femtoampere input bias op amp ADA4530-1
24-bit resolution ADC AD7172-2
USB interface to PC via SDP
Simple power supply: 9VDC input ADP7118, ADP2442, ADP7182
Measurement synchronization Trigger in/out signals
Can be reconfigured as electrometer front-end
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TIA
Ref
SDP
Power Management
ADCData
Isolator
Isolated Area
Shield
Trigger in
Trigger out
Input
Power
USB
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Example Application
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Detector PC
Measurement
Trigger
Measurement
Complete
Coaxial
Cable
USB
Photodiode
Faraday cup
Photomultiplier tubes (PMT)
Electron-multiplier
…
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-40
-20
0
20
40
60
80
100
0.001 0.01 0.1 1 10 100 1000 10000
SN
R (
dB
)
CURRENT (fA)
Maximum SNR vs. Current
0.1Hz
1Hz
10Hz
100Hz
1kHz
Prohibited by noise
Fundamental Measurement Limits
► The discrete nature of electrical current generates “shot noise”
► Shot noise increases as the square root of current
► At very low-level currents, shot noise can be greater than the measurement
Lower bandwidth; longer measurement times are required
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𝑖𝑖 = 2𝑞𝐼∆𝑓
𝑆𝑁𝑅 =𝐼
2𝑞∆𝑓
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► Option 1: resistor + buffer/amplifier
The current can flow through a resistor and then amplify
Increases burden voltage
Increases noise
► Option 2: transimpedance amplifier
The current still flows through a resistor
The op amp is used to reduce burden voltage
Keeps amplifier noise and error contribution low
Measuring Low-Level Currents
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► Resistor
► The greater the resistor, the better SNR Signal increases proportional to resistor
value (Ohm’s law)
Noise increases per square-root of resistor value (Johnson noise)
► Op Amp
► At low bandwidth, TIA has unity noise gain The op amp’s voltage noise contribution
to current noise gets divided by resistor value
-40.00
-20.00
0.00
20.00
40.00
60.00
80.00
0.1 1 10 100 1000 10000 100000
SN
R (
dB
)
TRANSRESISTANCE (MW)
SNR vs. TIA Resistor
100pARange
1pARange
Resistor and Op Amp Noise
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𝑖𝑟 =4𝑘𝑇
𝑅∆𝑓
𝑖𝑎 = 𝑒𝑎
𝑅
Op Amp
Noise
Resistor
Noise
Shot
Noise
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► In conclusion, to measure small
currents, use
A large resistor
A very low noise op amp
A very low input-bias op amp like the
ADA4530-1
► With large resistors The amplifier will become slower (due
to frequency compensation)
The op amp’s input bias current can saturate the output
Doubles every 10°C
► Example: 100pA*10GW = 1V
Resistor and Op Amp Noise
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The Implementation Problems
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Input bias current
Noise
Offset drift
Output swing
LeakageDielectric absorption
Surface contamination
Humidity
Radiated noise
RF
Power-line noise
Ground noise
LightTriboelectricity
Stability
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Noise Reduction: The Effect of Averaging vs. Filtering
► Sensitivity requires minimizing the noise
► Averaging and filtering reduce measurement bandwidth but work a little different
► The average of N samples: Reduces the noise by √N (flat-band
noise)
Reduces the effective maximum frequency by 2N
Increases measurement time
Rejects frequencies at the notches Good for power-line noise rejection
Produces aliasing above the notches
► Effectively a simple FIR filter Can be arbitrarily short/long
Very popular these days
► Sigma-delta ADCs like AD7172 perform this function internally using specialized filters
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1st Order Filter vs. Average
Example Filter Transfer Function in AD7172-2
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Flicker Noise and Low-Level Measurements
► Flicker (1/f) noise often limits measurement front-end sensitivity
► Noise reduction by filtering or averaging fails upon reaching the 1/f corner Averaging reduces flat-band noise
Noise amplitude is not reduced by √N after reaching this region
The longer the average, the longer we look at the signal, the lower we move on the frequency band
► Common flicker noise sources: ICs and semiconductor devices
Op amps, references, diodes…
Resistor excess noise Noise index sometimes available from
manufacturers
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Mochtenbacher and Connelly. Low-Noise
Electronic System Design. Wiley, 1993
1k
100
10
1 10 100 1k 10k 100k
NO
ISE
(n
V/
Hz)
FREQUENCY (Hz)
07
34
3-0
67
Flicker noise
regionWhite noise
region
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► Main sources:
Power lines (50/60Hz)
RF (e.g. wireless communications)
► Reaches the circuit via
Emission
Conduction
► Reduced by
Shielding
Minimize inductive loops (e.g. twist
wires)
Proper layout and grounding
Isolation
Power supply decoupling
Filtering
Extrinsic Noise
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Shielding
► Shields help keep stray fields
away from sensitive nodes
► Shields should be grounded when
exposed to operators for safety
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► Great for breaking ground loops, reduce noise
► But must bring data (ADuM3151) and power across isolation barrier
► Proper connections are required to avoid coupling of the common-mode current into the measurement
Or use batteries
► Shields can couple noise into isolated circuits
Keep shields away from sensitive nodes
Or use a driven shield (guard) inside the shield
Isolation
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► The surface of dielectrics has better conduction properties due to contamination, humidity, etc
This includes FR-4 and PTFE board materials, no matter how good their bulk properties are
► Sources of contamination:
Solder flux residue from assembly process
No-clean solder residue is difficult to remove
Dust and other particulate accumulation
► Washing assemblies is recommended after assembly
► Moisture reduces insulation properties of PCB and cables
Bake after wash to eliminate moisture absorption
Choose appropriate materials and perform measurements in controlled environments
Material Moisture Absorption
(%)
Hi Pref FR-4 0.50
Nearly pure PTFE 0.02
Surface and Board Leakage
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Source: Rogers Corp.
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► Better layout yields better
performance over time and
environmental conditions
► Guard rings
Prevents sensitive current to flow
through unwanted paths
Needs to be driven by an amplifier
at the same potential as the input
(e.g. a buffer)
► Remove solder mask
Eliminates the conduction path over
the guard ring
Solder mask absorbs moisture too
► Board cuts
Dealing with Leakage on the PCB
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Guard
Signal Trace
Leakage
Current
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Dielectric Absorption
► Caused by the polarization of the dielectric between conductor plates upon application of an electric field
► Commonly observed in
Capacitors
Multilayer PCBs
► The polarization relaxation has a longer time constant than the capacitance formed by the plates
► Often modeled as an RC in parallel with the “ideal” capacitor
► This is also why large capacitors are handled with a bleeding resistor or a short (safety)
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Dealing with Dielectric Absorption in PCBs
► Use PTFE (Teflon™)
laminates (Rogers Corp.)
► Stand-offs available for air-
wiring
► Minimize absorbing material:
board cuts
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Interconnects for Low-Current Measurement
► Cables and connectors Shielding is a must
► BNC, SMA and coaxial cables are OK as long as there is very little potential difference between center conductor and shield Cost effective
Some RF materials (PTFE) have also good low-leakage, low DA properties
Beware of safety with ground-isolated measurements
► Best: use triax connectors and cables BNC to triax adapters available
for interfacing to sensors
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Signal Guard
Shield (ground)
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Summary
Problem Cause Solution
Leakage on PCB Moisture and contamination Guard rings, board cuts, dust covers
Solder flux contamination Avoid no-clean solder; wash and bake
Dielectric
absorption
Charge trapped in dielectrics Use PTFE in boards and cables, guarding
Cable leakage Poor-quality insulation
between conductors
Use PTFE-insulated cables or triax cables
Extrinsic noise E/M fields, Powerline
interference
Shielding and guarding
Ground noise Isolation
Light-induced charge Shielding/covers
Mechanical vibrations,
triboelectricity
Cable tie-downs
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► Design of low-level current measurement hardware requires attention to many details!
► A femtoammeter module solves many problems and enables quick and simple prototyping and evaluation
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QUESTIONS?
THANK YOU!
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