Impedance matching for programmable gain amplifiers

• US 6,621,346 B1
• Filed: 05/19/1999
• Issued: 09/16/2003
• Est. Priority Date: 03/30/1998
• Status: Expired due to Term

• Current Assignee: Texas Instruments, Inc.
• Original Assignee: Texas Instruments, Inc.
• Inventors: Nabicht, Joseph T., Richardson, Donald C., Mukherjee, Subahashish, Islam, Kazi I.
• Primary Examiner(s): Choe, Henry
• Application Number:
  US09/314,764
• Time in Patent Office: 1,581 Days
• Field of Search: 330/144, 330/54, 330/82, 330/84, 375/222, 327/270, 327/308, 327/393
• US Class Current: 330/284
• CPC Class Codes: H04B 3/145 variable equalisers
• CPC Class Codes: H04B 3/23 using a replica of transmitted signal in the time domain, e.g. echo cancellers
• CPC Class Codes: H04L 25/03878 Line equalisers; line build-out devices
• CPC Class Codes: H04L 27/0002 analog front ends; means for connecting modulators, demodulators or transceivers to a transmission line duplex arrangements H04L5/143
• CPC Class Codes: H04L 5/023 Multiplexing of multicarrier modulation signals multicarrier modulation H04L27/2601

Abstract

Digital subscriber modems (8, 15) for use in Asynchronous Digital Subscriber Line (ADSL) communications are disclosed. The central office modem (8) includes a digital transceiver function (10) and an analog front end function (12), where the analog front end function (12) is integrated into a single integrated circuit. According to the disclosed embodiments, the analog front end function (12) includes a transmit and a receive side. On the receive side, an impedance matching circuit (56) is coupled to the input of a programmable gain amplifier (54C). The impedance matching circuit (56) is controlled by the same control signals (C1, C2, C3) as used to select the gain of the programmable gain amplifier (54C), so that a constant input impedance is presented to the signal input (RXP), independent of the selected gain.

Claims

1. A modem, comprising:

a host interface;

a digital transceiver function coupled to the host interface;

a line driver, for driving and receiving analog signals over a telephone facility; and

an analog front end circuit, comprising;

a digital interface, for communication of digital signals to the digital transceiver function;

an analog-to-digital converter, having an output coupled to the digital interface, and having an input; and

a programmable gain amplifier, having a signal input coupled to the line driver, having an output coupled to the analog-to-digital converter, and comprising;

an operational amplifier having an input and an output;

a plurality of input resistors, connected in series between the signal input and the input of the operational amplifier;

a plurality of gain switches, each having a conduction path connected between an associated pair of the plurality of input resistors and the input of the operational amplifier;

a plurality of matching resistors, connected in series between the signal input and a reference voltage; and

a plurality of matching switches, each having a conduction path connected between an associated pair of the plurality of matching resistors and the reference voltage, and each of the plurality of matching switches associated with one of the plurality of gain switches so that the state of each of the plurality of matching switches corresponds to the state of its associated one of the plurality of gain switches.

2. The modem of claim 1, wherein each of the plurality of gain switches has a control electrode for receiving a control signal;

wherein each of the plurality of matching switches has a control electrode for receiving a control signal,

and wherein the control electrode for each of the plurality of matching switches is connected in common with the control electrode of its associated one of the plurality of gain switches.

3. The modem of claim 2, wherein the analog front end further comprises:

control circuitry for generating the control signals.

4. The modem of claim 2, wherein each of the plurality of gain switches comprises a MOS transistor

and wherein each of the plurality of matching switches comprises a MOS transistor.

5. The modem of claim 1, wherein a first one of the plurality of gain switches has its conduction path connected between first and second ones of the plurality of input resistors, the first one of the plurality of input resistors being connected nearer the signal input than the second one of the plurality of input resistors;

wherein a second one of the plurality of gain switches has its conduction path connected between second and third ones of the plurality of input resistors, the second one of the plurality of input resistors being connected nearer the signal input than the third one of the plurality of input resistors;

wherein a first one of the plurality of matching switches is associated with the first one of the plurality of gain switches, and has its conduction path connected between first and second ones of the plurality of matching resistors, the first one of the plurality of matching resistors being connected nearer the reference voltage than the second one of the plurality of matching resistors;

and wherein a second one of the plurality of matching switches is associated with the second one of the plurality of gain switches, and has its conduction path connected between second and third ones of the plurality of matching resistors, the second one of the plurality of matching resistors being connected nearer the reference voltage than the third one of the plurality of matching resistors.