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The next 3 steps are one of the most important areas in the analog design domain. The requirements here vary widely, often necessitating application specific solutions. Fortunately a sea of more or less standardized components exist that can be used to implement this.
This step protects the rest of the circuit against outside influences. This is mainly EMS (electromagnetic susceptibility) en ESD (electrostatic discharge). Often over voltage protection is also included here.
In this step the signal is amplified or attenuated. The purpose is to bring the signal into the dynamic range of the A/D converter, and to filter out signals outside of the relevant frequency bands. These van be noise or other undesired signals, but also desired signals that lie out of the frequency range of the A/D converter (anti-aliasing).
An A/D converter transform an analog signal into a digital form. The input signal is most often a voltage and can be single ended or differential.The output signal is made available in one of the well known digital forms HCT or HC, but more and more often LVC. The signal can be output in parallel or serial form. In the latter case a protocol is use, like I2C or SPI or similar varieties. To be able to digitize the signal it is compared to a ruler: the reference. This part can be either internal or external to the A/D converter.
A large variety of A/D converters exist. All perform the same task, but still they can hardly be compared to each other in terms of current consumptions, speed, bandwidth, triggering, packaging etc.
And not to forget price. There is probably not another type of component to be found with such a large spread in price.
After A/D conversion, the original signal is irreversibly distorted by quantization (rounding errors) and sampling (the conversion takes place at discrete moments, the information between these moments is lost). When the signal has been conditioned in the appropriate manner, the latter does not need to be a problem. The quantization errors however can only be kept samll enough be applying an A/D converter with sufficient resolution (the number of bits), averaging and rounding error later is impossible.
Transforming to the time domain
In many cases it is also possible to transform the analog (voltage) input signal to a signal in the time domain. No quantization needs to take place at this time (but obviously some for of sampling does). When the time domain signal is for the rest digital (i.e. HCT or LVC compatible), it can be directly connected to for instance a microcontroller. The latter can then easily measure frequency or period of the signal.
This has a number of important advantages:
- The dynamic range is the time domain is much larger and generally spans 1 ms to 10 s.
- The resolution is determined by the clock of the frequency meter. This can be the microcontroller or an external fast counter.
- Successive periods of the signal can be averaged since the quantization error is not correlated with the signal. This is not possible with an ordinary A/D converter.
- The signals in the time domain are more robust then analog signals and can be easily transported over larger distances.
- A certain standardization of the signal conditioning can be realized. The flexibility is maintained by performing certain functions in software.
For all of the above cases Smartec's Universal Transducer Interface (UTI) has been developed.
The UTI is an ASIC marketed by the Nederland based firm Smartec. This IC can be considered to be an oscillator of which the period depends on the measurand. It allows us to do AC measurements that are filtered for LF (low frequent) and HF (high frequent) disturbances. The measurements are also automatically compensated for offset and scale errors. The AC measurements take place in the band around 20 kHz which is far from mains disturbances (50 Hz - 2 kHz) as well as from microcontroller clock signals and power supply switching (> 100kHz).
The UTI output signal is low frequent TTL or LV CMOS compatible signal with periods between 10 ms and 50 ms, which can easily be transported over longer distances. The various periods correspond to the4 successive sensor, offset and scale measurement.
Sensors that are supported by this IC are based on:
- Resistance bridges
By the use of an AC measurement the UTI is especially suited to be placed near to the sensor, for instance in pressure sensors or capacitive level gauges.
Sometimes it is inevitable to place the signal conditioning at a certain distance from the sensor, for instance because the sensor is placed in and environment that is not suited for electronic components (high temperature, aggressive chemicals). By adding a small amount of preconditioning (consisting of a buffer and a chopper) effectively a DC measurement can be performed.
This trick can also be applied with sensors that produce a DC output voltage, like thermocouples.
Compared to a conversion in the voltage domain using a 24 bits A/D converter, the UTI with chopper can still give a cost effective solution.
Exalon Delft has years of experience in the field of industrial signal conditioning and A/D conversion. Traditional with 8 to 24 bits A/D converter ór UTI application, we have got it for you or we will develop it quickly.
Our expertise spans:
- Segmented capacitive level measurement. The 0.15 pF capacitance is measured with 0.1 fF (1 fF = 0.001 pF) accuracy
- Grounded capacitive sensor measurement. The 500 pF capacitance is measured with 0.5 pF accuracy.
- Thermocouple measurement. The +/- 12 mV voltage is measured with 1 mV accuracy.
- Pt100 measurement. The approx. 100 Ohm resistance is measured with 10 mOhm accuracy via 100 m long cabling.