| 摘要 |
A continuous time electrical filter fabricated as an integrated circuit includes capacitors (CO,CN) and resistors (R1,R2). Since capacitors and resistors are difficult to integrate with accurately defined values a trimming circuit is provided which operates switches (S1-SN) to select appropriate ones of the capacitors (CN) to accurately define the cut-off frequency of the filter. The trimming circuit comprises a capacitor (TC2) which is charged through a resistor TR1 during a first period and which is discharged in incremental steps by a capacitor (TC1). The number of incremental steps is counted by a counter (11) and transferred to a register (13). The outputs (S1-SN) of the register control the switches (S1-SN). Instead of adjusting the value of the capacitors in the filter, the values of the resistors may be adjusted. If this is done a convenient procedure is to short out selected portions of the resistors. More than one capacitor or resistor may be adjusted using a single counter and register.
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