Op Amp Considerations for Multiplexed Data Acquisition Applications

Sunday, September 16th, 2018 - Digital Electronics

Op Amp Considerations for Multiplexed Data Acquisition Applications

Multiplexing is a fundamental part of many data acquisition systems. Switches used in multiplexed data acquisition systems are generally CMOS-types. Utilizing P-Channel and N-Channel MOSFET switches in parallel minimizes the change of on-resistance (RON) as a function of signal voltage. On-resistance can vary from less than five to several hundred ohms depending upon the device. Variation in onresistance as a function of signal level (often called RON-modulation) causes distortion if the multiplexer drives a load, therefore RON flatness is also an important specification.

Because of the effects of non-zero RON and RON-modulation, multiplexer outputs should be isolated from the load with a suitable buffer op amp. A separate buffer is not required if the multiplexer drives a high input impedance, such as a PGA, SHA or ADC— but beware! Some SHAs and ADCs draw high frequency pulse current at their sampling rate and cannot tolerate being driven by an unbuffered multiplexer.

Op Amp Considerations for Multiplexed Data Acquisition ApplicationsBasic CMOS analog switch

Key multiplexer specifications are switching time, on-resistance, on-resistance flatness, and off-channel isolation, and crosstalk. Multiplexer switching time ranges from less than 20ns to over 1µs, RON from less than 5Ω to several hundred ohms, and off-channel isolation from 50 to 90dB.

A number of CMOS switches can be connected to form an analog multiplexer. The number of input channels typically ranges from 4 to 16, and some multiplexers have internal channel-address decoding logic and registers, while with others, these functions must be performed externally. Unused multiplexer inputs must be grounded or severe loss of system accuracy may result. In applications requiring an op amp buffer, it should be noted that when the multiplexer changes channels it is possible to have a full scale step function into the op amp and the ADC which follows it.

Op amp settling time must be fast enough so that conversion errors do not result. It is customary to specify the op amp settling time to 1 LSB, and the allowed time for this settling is generally the reciprocal of the sampling frequency.

Typical multiplexed data acquisition system requires fast settling op amp buffer

Typical multiplexed data acquisition system requires fast settling op amp buffer

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