ADS1158: EXTERNAL MULTIPLEXER LOOP

Hi Shreyas,

The MUXOUT/ADCIN pins allow you to design and connect your own signal conditioning circuit for filtering/level-shifting/amplifying/buffering the input signal(s) to the ADS1158. By connecting this circuit after the MUX, you can potentially use a single amplifier circuit for buffering all 16-input channels.

A signal conditioning circuit is not always needed; however, the two most common use cases for this circuit are 1) to buffer the input signals and 2) for converting single-ending input signals to a fully-differential signals.

1. Buffering input signals into the ADC is especially important if your signal source is not already buffered or if the output impedance of your signal source is high. The input impedance of a delta-sigma modulator is not all that high (typically somewhere between 10-100 kOhms). Adding a buffer signification improves the input impedance (as seen by the signal source) and reduces "loading" effects on the signal. For example, if the output impedance of a given source is 1 kOhm, and the input impedance of the ADC is 100 kOhm, then there is effectively a resistor divider causing the input signal to the ADC to droop by almost 1%.
   
   
2. Most delta-sigma ADCs measure differential voltage between two inputs, and return a two's compliment signed output code with the measurement result. The ADS1158 is no exception, as even the single-ended inputs are measured as the potential difference between AINx and AINCOM. If a single-ended (S.E.) input voltage is measured with respect to ground then only the positive input range of the ADC will be utilized. To correct for this and increase the input range utilization, you must either bias AINCOM to a mid-supply voltage, such that the differential voltage "AINx - AINCOM" can span from -2.5V to 2.5V (using a ~2.5V reference) OR the input signal conditioning will need to convert the 0-5V signal to a +/- 2.5V or +/- 5V signal (depending on whether a 2.5V or 5V reference is used).
    

NOTE: In Fixed-Channel mode, the ADS1158's "MUXSCH" register does not have values to select AINCOM as one of the MUX input pins. AINCOM is only used in Auto-Scan mode. Therefore, if you use the AINCOM input pin, you will probably always be using the ADS1158 in Auto-scan mode, OR alternatively, you could short AINCOM to AIN15 and then use Fixed-channel mode to measure single-ended inputs on pins AIN0 through AIN14.