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Frequently Asked Questions
General QuestionsWhich filter type should I use for time-domain analysis?The Bessel filter type is best suited for time-domain applications requiring minimum distortion of rapid slope changes. All cards in the AAF line offer a Bessel response. The Bessel features a uniform group delay and the lowest wideband noise, but it has a drooping amplitude response in the frequency domain and a gentler cutoff frequency than other filter types. The high-speed linear phase filter type also is recommended for time domain applications. It has the highest maximum cutoff frequency and a sharper cutoff than Bessel, but its input/output voltage limits are reduced. On the AAF-3, the standard linear phase is best suited for time-domain applications. It offers automatic DC offset compensation. The optional Bessel filter type for the AAF-3 does not offer the automatic compensation, but offers a higher maximum cutoff frequency (with mode B) of 66kHz, versus 50kHz for the standard AAF-3 linear phase. What software is provided with each board and what third-party software can your boards be used with?The AAF-3, AAF-16 and PGA-16 (not the AAF-1) come with free menu-driven and driver-level software for DOS, Windows 3.1/95/NT and LabVIE 3.x/4.x. Our boards also can be used with any third-party software - such as Labtech Notebook - if that software can call an external DOS subroutine library or a Windows dynamic link library (DLL). The AAF-3, AAF-16, and PGA-16 come with such linkable libraries, which provide more than 10 routines for configuring and controlling our filter and amplifier cards. We provide 16-bit DLLs for Windows 3.1 and 32-bit DLLs for Windows 95 and NT. AAF-1How do you set the AAF-1 filter cutoff frequency?With the AAF-1, which is the only board in the AAF line that is not software-programmable, you have two options. Unless you have an application that requires software programmability, the built-in, manually controlled clock source on the AAF-1 frequently serves as the most convenient and economical way to set the filter cutoff frequency. The built-in clock source can be adjusted for cutoff frequencies between 5 Hz and 200 kHz (to 0.1 Hz with an external clock), with the specific range depending on which filter type you have purchased. A switch with overlapping ranges selects the frequency range. A potentiometer fine tunes the cutoff from outside the computer cabinet and is accessible when the cover is closed. For some applications, you may want to use an external source for the digital clocking waveform. The external source is connected to the AAF-1 by the FREQ IN pin on the AAF-1 connector. The source must be set at the frequency calculated from the filter type constant multiplied by the desired cutoff frequency. A standard counter-timer board can serve as an external clock source. For example, the model CTM-05 counter-timer board from Keithley Metrabyte has five independent counters, one or all of which can be used to set AAF-1 cutoff frequencies. This approach is especially useful when the cutoff frequencies are not very high. AAF-3What resources of the PC does the AAF-3 use?The AAF-3 uses 4 register locations from the base address, which is setable between 200h and 3FEhex. The AAF-3 does not use any interrupts. Are the filter outputs differential or single-ended?With the standard elliptic and linear phase filter types, the AAF-3 filter output is single-ended. You can achieve differential input by: a) using the on-board instrumentation amplifier channels, or b) using one of the five optional filter types (Bessel, Butterworth, Cauer, high-speed Cauer, high-speed linear phase.) Is the amplifier before or after the filter?The amplifier is before the filter. If the gain is directly on the signal (pre-filter gain), only the signal is amplified. Pre-filter gain, which the PGA-16 provides, can make the low-level signal override the measurement-system noise, providing a dramatic improvement in signal-to-noise ratio. Post-filter gain (gain on the A/D board) cannot, since it amplifies the measurement-system noise along with the signal. AAF-16What are the input and output signal voltage ranges?Both input and output are +/-10V. Is it possible to control the cut-off frequency using an external clock?For special purposes, such as tracking filters, the cutoff frequency of an AAF-16 filter channel may be controlled by an external clock source. Provision is made for two independent external clock inputs, in addition to the two internal programmable clocks. The clock frequency controls the cutoff frequency of the filter(s) to which it is programmed. The external clock should be a TTL- or CMOS-logic-level square wave with symmetry of roughly 50%. If the driver is TTL, a pull-up resistor to +5 volts will improve the voltage margins. Rise and fall times must each be less than 10% of the clock cycle and in no event greater than 1 microsecond. Can you supply a connector cable between my A/D board and AAF-16?Yes, we can build a custom cable that directly connects the two boards. Part numbers are:
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