High Amplitude Arbitrary/Function Generator Simplifies Measurement in Automotive, Semiconductor, Scientific and Industrial Applications Application Note
Figure 11. Measurement set-up to characterize amplifier.
The sensitivity of the TCD depends on the current flow through the heated elements and the temperature inside the cells. Higher currents increase the voltage change across the measurement bridge and result in higher temperatures, but can also shorten the filament life. These trade-offs need to be factored in when laying out the electrical design. In practice, the output voltage of the bridge is typically between 15 V and 20 V.
The output of the measurement bridge is connected to an amplifier. A resistor network at the amplifier input enables the selection of different sensitivity ranges. At the amplifier output, an analog-to-digital converter samples the signal and con- verts it into digital form for processing by a digital processor.
The designer of the measurement amplifier needs to charac- terize the amplifier for its bandwidth, slew rate, step response, linearity and dynamic range. This requires a variety of input
signals of different wave shapes, frequencies and amplitudes. It would be impractical to generate all these input signals via actual measurements with the TCD. Using a modern arbitrary/function generator to simulate the signal from the Wheatstone bridge is much more convenient and offers more flexibility. Figure 11 shows the measurement setup. An oscilloscope measures the amplifier output.
If the output of the arbitrary/function generator is limited to maximum amplitudes of 10 Vpp into 50 Ohm, a separate amplifier is required to boost the signal level to the 15 to 20 V typically delivered by the Wheatstone bridge. Using an arbi- trary/function generator like the Tektronix AFG3011 that is capable of delivering these high amplitude levels directly sim- plifies the measurement setup. It also gives the user direct visibility and control over the effective amplitude of the test signal that feeds into the amplifier.