fs t acq +t conv sampling rate of 100kSPS is possible, if all the channels are addressed. The per d a The AD1674 has a conversion time of 9us, an acquisition time of lus to 12-bits channel sampling rate is obtained by dividing the adC sampling rate by m FILTERING CONSIDERATIONS IN DATA ACQUISITION SYSTEMS Filtering in data acquisition systems not only prevents aliasing of unwanted signals but also reduces noise by limiting bandwidth In a multiplexed system, there are basically two places to put filters: in each channel, and at the multiplexer output FILTERING IN A DATA ACQUISITION SYSTEM MUX PGA> LPF SHA CH M For Sequential Sampling, fe,< Figure 6.9 The filter at the input of each channel is used to prevent aliasing of signals which fall outside the Nyquist bandwidth. The per- channel sampling rate(assuming each channel is sampled at the same rate)is fs/M, and the corresponding Nyquist frequency is fs/2M. The filter should provide sufficient attenuation at f/2M to prevent dynamic range limitations due to aliasing. A second filter can be placed in the signal path between the multiplexer output and the adC, usually between the pga and the Sha. The cutoff frequency of this filter must be carefully chosen because of its impact on settling time. In a multiplexed system such as shown in Figure 6.7, there can be a fullscale step voltage change at the multiplexer output when it is switched between channels. This occurs if the signal on one channel is positive fullscale, and the signal on the adjacent channel is negative fullscale. From the timing diagram shown in Figure 6.8, the signal from the filter has essentially the entire conversion period (1/fs) to settle from the step voltage. The signal should settle to within ILSB of the final value in order not to introduce a significant error. The settling time requirement therefore places a lower limit on the filters cutoff frequency. The single- pole filter settling time required to maintain a given accuracy is shown in Figure 6.10. The settling time requirement is8 fs t acq t conv £ + 1 The AD1674 has a conversion time of 9µs, an acquisition time of 1µs to 12-bits, and a sampling rate of 100kSPS is possible, if all the channels are addressed. The per￾channel sampling rate is obtained by dividing the ADC sampling rate by M. FILTERING CONSIDERATIONS IN DATA ACQUISITION SYSTEMS Filtering in data acquisition systems not only prevents aliasing of unwanted signals but also reduces noise by limiting bandwidth. In a multiplexed system, there are basically two places to put filters: in each channel, and at the multiplexer output. FILTERING IN A DATA ACQUISITION SYSTEM Figure 6.9 The filter at the input of each channel is used to prevent aliasing of signals which fall outside the Nyquist bandwidth. The per-channel sampling rate (assuming each channel is sampled at the same rate) is fs /M, and the corresponding Nyquist frequency is fs /2M. The filter should provide sufficient attenuation at fs /2M to prevent dynamic range limitations due to aliasing. A second filter can be placed in the signal path between the multiplexer output and the ADC, usually between the PGA and the SHA. The cutoff frequency of this filter must be carefully chosen because of its impact on settling time. In a multiplexed system such as shown in Figure 6.7, there can be a fullscale step voltage change at the multiplexer output when it is switched between channels. This occurs if the signal on one channel is positive fullscale, and the signal on the adjacent channel is negative fullscale. From the timing diagram shown in Figure 6.8, the signal from the filter has essentially the entire conversion period (1/fs ) to settle from the step voltage. The signal should settle to within 1LSB of the final value in order not to introduce a significant error. The settling time requirement therefore places a lower limit on the filter's cutoff frequency. The single-pole filter settling time required to maintain a given accuracy is shown in Figure 6.10. The settling time requirement is