(Detector Array Operating) Electronics
Main tasks of a detector array electronics are
- to provide the clock frequency fClock to drive the chip’s internal multiplexer,
- to initiate this multiplexer to start the readout,
- to handle the EndOfScan (EOS) signal in case of trigger demand,
- to process the small signals generated by the detector, including amplification,
- priming the signal for the ADC, such as integration, sample & hold
- the conversion, plus some
- buffering for the
- data transfer to the host computer. Furthermore,
- trigger input signals have to be processed to start the readout on demand.
The electronics can also be used to control periphery devices, e.g. by I2C bus (light sources, shutters, stepping motors, etc.).
The readout principle of a detector array can be easily understood envisioning capacitors which are charged by light influx. By successively closing a switch to the capacitor, i.e. connecting it to a sampling line, the charge of one pixel after the other is drained, the area under the exponentially decharging curves reflects the number of photons detected.
Each readout is generated by a START pulse. The time between to readout processes is the exposure or integration time, i.e. it can be easily controlled by the time distance of two consecutive START pulses.
The key feature influenced most by the electronics is the low-signal level noise, for this is often dominated by the readout noise. The accuracy with which an analog signal can be detected depends partly on the time tPixel available to analyze the signal:
tPixel = 1/ fClock
This is directly related to the clock frequency applied. This frequency defines also – in combination with the number of pixels – the overall time tArray required to read out an array (with N pixels), i.e. to acquire a spectrum:
tArray = NPixel/ fClock
Therefore, the faster the readout the less time per pixel, the less accurate the signal. The designer’s challenge is to find the best compromise. Back to top▲
CCD readout noise across chip (Hamamatsu S7031), with 16 bit converter
It also has to be taken into account that the chip temperature rises with an increase of the clock frequency.
The overall measurement time, i.e. the time to acquire a spectrum is:
tAcqu. = tRead + tInt
The procedure and thus the timing various with the chip type a little. Back to top▲
Separate Read Phase: With some InGaAs and some CMOS chips, the exposure phase and the readout phase are separated, i.e. the sensor array is not sensitive during the readout process. The advantage of this is that very hsort exposure periods are possible and that all pixels are illuminated during the exact same time period, thus all wavelength info is acquired at the very same time. The disadvantage, that the overall light collection is reduced by the time required for the readout.
For timing details please see Measurement Speed.
The synchronization of the spectrometer readout to external processes is a demanding task, especially when external events have to control the data acquisition, e.g. a sample on a conveyor belt has to be measured when the sample is under the measurement head: a trigger signal derived from the sample has to start the measurement process. the EndOfScan signal, generated by the detector array after the last pixel is read, can be used to trigger a flash source, which confines all the optical energy to the moment the sample has to be measured. The next readout of the sensor array contains the spectral information. Back to top▲
Electronic Signal Chain: The signal directly out of a sensor chip is very weak. A preamplifier as close to the chip as possible boost the signal. In preparation for the digitization, an integrator and Sample & Hold circuitry are priming the signal further. After the Analog-to-Digital Converter, the signal is buffered and transferred to the controller or host computer.
The operating electronics also takes care of providing the clock and START signals to make the detector chip work, to handling the EOS signal, to provide further I/Os as well as to store sensor relevant parameters. Last but not least, it makes sure the various supplied voltages are available as stable as required.
To adapt to specific measurement tasks a variety of so-called operating modes have been created. To learn more please contact us. Back to top▲