Time_domain_bridge

Published:2009/7/24 5:31:00 Author:Jessie | From:SeekIC

Figure 4-22 shows a bridge circuit that operates in the time domain, and is particularly suitable for capacitance measurement. With S1 closed, the comparator output is high. When S1 opens, capacitance Cx charges. When the Cx potential crosses the voltage established at the comparator noninverting input, the comparator output goes low. The elapsed time between the switch opening and the comparator going low is proportionate to the Cx, value. The circuit is insensitive to supply and repetition rate variations, and can provide good accuracy if the time constants are kept much larger than comparator and switch delays. The LT1011 delay is about 200 ns, and the LTC201A delay is about 450 ns. To ensure 1% accuracy, the time constant of Cx and the series resistor should not go below 65 μs.Extremely low values of capacitance might be influenced by switch charge injection.In such cases, switching should be implemented by alternating the bridge drive between ground and +15 V. LINEAR TECHNOLOGY, APPLICAION NOTE 43, P. 27.



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