Low-pass filter with a summing function

• US 6,369,645 B1
• Filed: 09/09/1998
• Issued: 04/09/2002
• Est. Priority Date: 09/09/1997
• Status: Expired due to Fees

• Current Assignee: NEC Electronics Corporation (Renesas Electronics Corporation)
• Original Assignee: NEC Corporation
• Inventors: Takahashi, Yutaka
• Primary Examiner(s): Le, Dinh T.
• Application Number:
  US09/150,298
• Time in Patent Office: 1,308 Days
• Field of Search: 327/552, 327/558, 327/361, 327/344, 327/553, 327/554
• US Class Current: 327/558
• CPC Class Codes: H03H 11/126 using a single operational amplifier H03H11/1204 takes precedence; parallel-T filters H03H11/1295

Abstract

In a low-pass filter (10) having a filter input terminal (10-i1) supplied with a filter input signal (V₁) and comprising an operational amplifier (11) and other peripheral passive elements (R_in, C_in, R_f, C_f, R_1-1), the low-pass filter (10) further has an additional filter input terminal (10-i2) supplied with an additional filter input signal (V₂) and comprises an additional filter input resistor (R_1-2) connected between the additional filter input terminal (10-i2) and a common node (a₀). The operational amplifier (11) has an inverting input port (11-1), a noninverting input port (11-2) connected to the reference voltage terminal (12), and an amplifier output port (11-3) for producing an amplifier output signal. Connected to the amplifier output port (11-3) of the operational amplifier (11), a filter output terminal (10-o) produces the amplifier output signal as a filter output signal. Thereby, the low-pass filter (10) sums up the filter input signals (V₁, V₂) to obtain a summed signal and removes a high frequency component from the summed signal to make the filter output terminal (10-o) produce a low-pass filtered signal as the filter output signal.

Claims

1. A low-pass filter comprising:

first through N-th positive filter input terminals supplied with first through N-th positive filter input signals, respectively, where N represents a first positive integer which is not less than two;

first through M-th negative filter input terminals supplied with first through M-th negative filter input signals, respectively, where M represents a second positive integer which is not less than two;

an operational amplifier having inverting and noninverting input ports, and inverting and noninverting amplifier output ports for producing inverted and noninverted amplifier output signals, respectively;

a first amplifier input resistor having an end connected to the noninverting input port of said operational amplifier and another end connected to a positive common node;

a second amplifier input resistor having an end connected to the inverting input port of said operational amplifier and another end connected to a negative common node;

an amplifier input capacitor connected between the positive and the negative common nodes;

a first filter feedback resistor connected between the positive common node and the inverting amplifier output port of said operational amplifier;

a second filter feedback resistor connected between the negative common node and the noninverting amplifier output port of said operational amplifier;

a first filter feedback capacitor connected between the noninverting input port of said operational amplifier and the inverting amplifier output port of said operational amplifier;

a second filter feedback capacitor connected between the inverting input port of said operational amplifier and the noninverting amplifier output port of said operational amplifier;

first through N-th positive input impedance circuits connected between the positive common node and said first through said N-th positive filter input terminals, respectively;

first through M-th negative input impedance circuits connected between the negative common node and said first through said M-th negative filter input terminals, respectively;

an inverting filter output terminal, connected to the inverting amplifier output port of said operational amplifier, for producing the inverted amplifier output signal as an inverted filter output signal; and

a noninverting filter output terminal, connected to the noninverting amplifier output port of said operational amplifier, for producing the noninverted amplifier output signal as a noninverted filter output signal.

2. A low-pass filter as claimed in claim 1, wherein said first through said N-th positive input impedance circuits comprise first through N-th positive filter input resistors connected between the positive common node and said first through said N-th positive filter input terminals, respectively, said first through said M-th negative input impedance circuits comprising first through M-th negative filter input resistors connected between the negative common node and said first through said M-th negative filter input terminals, respectively.