MULTIPLEXED DATA AQUISITION SYSTEM WITH PGA AND SAR ADC CONVERT COMMAND MUX PGA SAR ADC NO SHA) LPF a Example: M N- 12 and tconv *2Dsec, Then fin shAlin Figure 6.6 In a multiplexed system it is possible to have a positive fullscale signal on one channel and a negative fullscale signal on the other. When the multiplexer switches between these channels its output is a fullscale step voltage. All elements in the signal path must settle to the required accuracy(ILSB) before the conversion is made. The effect of inadequate settling is dc crosstalk between channels The SAR ADC chosen in this application has no internal sHa(similar to the industry-standard AD5 74-series), and therefore the input signal must be held constant (within 1LSB)during the conversion time in order to prevent encoding errors. This defines the maximum rate-of-change of the input signal: 1 LSB max conv The amplitude of a fullscale sinewave input signal is equal to(2 N)/2, or 2(N-1), and its maximum rate-of change is 2nf max -1 Ifmax 2 dt max Setting the two equations equal, and solving for fmax. π·2 t conv For example, if the ADC conversion time is 20usec(corresponding to a maximum sampling rate of slightly less than 50kSPS), and the resolution is 12-bits, then the6 MULTIPLEXED DATA AQUISITION SYSTEM WITH PGA AND SAR ADC Figure 6.6 In a multiplexed system it is possible to have a positive fullscale signal on one channel and a negative fullscale signal on the other. When the multiplexer switches between these channels its output is a fullscale step voltage. All elements in the signal path must settle to the required accuracy (1LSB) before the conversion is made. The effect of inadequate settling is dc crosstalk between channels. The SAR ADC chosen in this application has no internal SHA (similar to the industry-standard AD574-series), and therefore the input signal must be held constant (within 1LSB) during the conversion time in order to prevent encoding errors. This defines the maximum rate-of-change of the input signal: dv dt LSB max t conv £ 1 The amplitude of a fullscale sinewave input signal is equal to (2^N)/2, or 2^(N-1), and its maximum rate-of change is dv dt max 2 f max 2 N 1 fmax 2 N = × - p = p × Setting the two equations equal, and solving for fmax, f N t conv max £ × 1 p 2 For example, if the ADC conversion time is 20µsec (corresponding to a maximum sampling rate of slightly less than 50kSPS), and the resolution is 12-bits, then the