Introduction
The TZA500 from Artifex Engineering is a precision transimpedance amplifier (TZA or TIA) used to convert very small currents (pA–mA) from sensors such as photodiodes or PMTs into measurable voltages. Because these measurements often occur at extremely low signal levels, noise suppression may be critical for obtaining accurate results.
Below is a detailed explanation of the TZA- Best Methods for Noise Suppressionavailable when using the TZA500, including inherent design, instrument-based techniques and experimental setup methods.
Table of Contents
Noise Rejection by Design
Electromagnetic Interference (EMI)
The TZA series amplifiers are designed to be largely insensitive to electromagnetic interference, which is important in industrial environments.
Design features contributing to this include:
- differential amplifier topology (option)
- low-impedance input node
- short internal signal paths
- shielded housing
These design choices reduce susceptibility to:
- RF pickup
- ground loops
- switching noise from nearby electronics.
Ground Isolation
The TZA500 interface includes separate analog and digital grounds, which should not normally be connected together externally.
Purpose:
- prevents digital switching noise from coupling into analog measurement signals
- eliminates ground loop noise
Separating these grounds reduces disturbances caused by digital communication signals.
Compact Source-Mounted Operation
The TZA500’s compact design allows it to be placed close to the sensor, minimizing cable length.
Benefits:
- reduced capacitive noise
- less EMI pickup
- improved stability
This is especially useful with photodiodes or ionization detectors.
Instrument Features for Noise Suppression
Differential Input Noise Cancellation
One of the most powerful noise suppression mechanisms in the TZA500 is the differential input stage option.
Principle
In differential mode, both terminals of the current source are actively measured by two amplifier inputs. The signals are then subtracted internally.
Result:
Because the return path of the current is also measured, the subtraction process doubles the signal but cancels identical noise components.
- Signal components remain
- Noise common to both inputs is cancelled
This removes common-mode noise, which typically originates from:
- electromagnetic interference (EMI)
- power supply noise
- environmental electrical pickup
Typical applications
- Photodiode measurements in noisy environments
- Remote detectors with long cables
- Industrial optical monitoring
Twisted-Pair Differential Cabling (BR2 Connector)
For remote current sources, the TZA500 supports a BR2 connector, which supports use of a shielded twisted pair cable.
Benefits:
- magnetic field noise cancels in the twisted pair
- reduced pickup from external interference
- better signal integrity over longer cables
This significantly reduces noise when measuring remote detectors.
The effectiveness of this technique is illustrated in our document “Comparison of Noise Levels of TZA500 in BNC and BR2 versions”.
Auto-Null (Offset and Dark Current Suppression)
The TZA500 includes an auto-null function that can compensate offsets up to about 10V equivalent at the output. This function may be activated via software (under USB control) or hard switched when controlling the device via the hardwire interface.
Purpose
This function removes DC offsets caused by:
- photodiode dark current
- amplifier input offset
- background optical power
Working principle
This function stores the measured signal at the moment of activation of the function and subtracts this value from all subsequent measurements.
For the application of background suppression:
- Block or turn off the source to be measured.
- Start the measurement and activate the auto-null function.
- Unblock or turn on the source to be measured.
Note that this solution is only effective, if the background offset is constant. Background sources such as a photodiode signal with room lighting typically fluctuates at 100Hz or 120Hz which will therefore be imperfectly suppressed.
Advantages
- Allows detection of small signal variations
- Improves dynamic range
Typical uses
- detection of small signal changes around a DC background
Bandwidth Limiting (Low-Pass Filtering)
Noise in electronic systems typically increases with bandwidth.
The TZA500 therefore allows bandwidth reduction to suppress high-frequency noise. The bandwidth setting may be selected via software (under USB control) or hard switched when controlling the device via the hardwire interface.
Available bandwidth settings: 10kHz (full bandwidth), 1kHz, 100Hz, 10Hz
Reducing bandwidth suppresses high-frequency noise components and smooths the output signal.
Noise–bandwidth relation
Where B = bandwidth.
Thus, reducing bandwidth by a factor of 100 reduces noise by roughly 10×. Note that this method of noise reduction comes at the cost of increased measurement time.
Example
If measuring slowly varying optical power (e.g., laser drift):
- Set bandwidth to 10–100 Hz
- Signal becomes much more stable.
Gain Optimization
The TZA500 provides six selectable gain ranges, covering currents from mA down to nA levels.
Correct gain selection is critical for noise performance.
Too low gain
- signal small relative to ADC resolution → poor signal-to-noise ratio (SNR)
Too high gain
- amplifier saturation
Best practice
Choose the gain such that:
This uses most of the amplifier dynamic range without clipping. When using the graphical user interface (GUI) supplied with the instrument, the auto-gain mode may be selected which ensures that the amplifier output remains within this recommended range.
Digital Averaging and Sampling Control
The TZA500 digitizes signals with an onboard ADC and can stream measurements at about 1.5 kS/s.
Noise can be reduced by digital averaging:
Whereby N is the number of data points contained in the average. The noise reduction then scales as:
Example:
Samples averaged | Noise reduction |
4 | 2× |
16 | 4× |
100 | 10× |
Mechanical and Optical Noise Reduction (External Methods)
Although not part of the amplifier electronics, several setup techniques strongly influence noise levels.
Short detector cables
Long cables increase capacitance and leading to microphonic noise pickup.
Optical shielding
For photodiodes:
- remove ambient light
- eliminate stray reflections
- use optical filtering if possible
Temperature stabilization
Dark current and amplifier offsets drift with temperature.
Proper grounding
Use a single-point ground system to avoid loops.
Summary of Noise Suppression Methods
Method | Noise Type Suppressed |
Differential input | Common-mode electrical noise |
Auto-null | Dark current and DC offset |
Bandwidth limiting | High-frequency electronic noise |
Gain optimization | Improves SNR |
Twisted-pair BR2 cables | EMI pickup |
Ground isolation | Digital/ground loop noise |
Instrument design | EMI rejection |
Digital averaging | Random noise |
In practice, the most effective combination is:
- Differential input
- Auto-null to remove dark current
- Reduced bandwidth (10–100 Hz)
- Proper gain selection
- Shielded twisted-pair cables
Together these can improve measurement stability by orders of magnitude when measuring picoamp-level signals.
About the Author:
Donovan Ellis
Product Specialist & Sales
Donovan Ellis studied computer engineering in Hamilton, Ontario, and today works as Artifex Engineering’s sales and product specialist for measuring instruments. Through daily contact with customers, he knows their questions and challenges firsthand — from designing and planning to calibrating and building devices, he understands the everyday problems and tricks of the trade.
