Method and structure for improving the linearity of MOS switches

• US 6,897,701 B2
• Filed: 05/13/2003
• Issued: 05/24/2005
• Est. Priority Date: 05/13/2003
• Status: Active Grant

• Current Assignee: Texas Instruments, Inc.
• Original Assignee: Texas Instruments, Inc.
• Inventors: Betty, Christopher L., Richardson, Donald C., Chen, Feng
• Primary Examiner(s): Zweizig, Jeffrey
• Application Number:
  US10/436,769
• Publication Number:
  US 20040227565A1
• Time in Patent Office: 742 Days
• Field of Search: 327/307, 327/333, 327/382, 327/530, 327/534, 327/535, 327/540
• US Class Current: 327/307
• CPC Class Codes: G05F 3/262 using field-effect transistors only
• CPC Class Codes: H03K 17/063 in field-effect transistor switches

Abstract

A technique is provided to linearize a MOS switch on-resistance and the nonlinear junction capacitance. The technique linearizes the sampling switch by using a buffer having substantially unity gain with proper DC shift to drive an isolated bulk terminal of the MOS well to improve the spurious free dynamic range (SFDR). In this way, the 2nd-order effect such as nonlinear body effect (V_T(V_SB)) and nonlinear junction capacitance (C_j(V_SB)) can be substantially removed.

Claims

1. A method of linearizing a MOS-based switching circuit, the method comprising the steps of:

providing a switching circuit having at least one MOS switch and a buffer having substantially unity gain and a DC voltage offset, wherein the at least one MOS switch comprises a source, a gate, a drain and a bulk terminal; and

driving at least one MOS switch bulk terminal via the buffer in response to an input signal to provide a level-shifted replica of the input signal such that a desired DC voltage drop reverse biases at least one MOS switch source-bulk junction to control the spurious free dynamic range of the MOS-based switching circuit.

2. The method according to claim 1 wherein the step of driving at least one MOS switch bulk terminal via the buffer comprises driving the at least one MOS switch bulk terminal via the buffer in response to the input signal to substantially remove 2nd-order effects such as body effect (V_T(V_SB)) and nonlinear junction capacitance (C_j(V_SB)) associated with the MOS switch.

3. The method according to claim 1 wherein the step of driving the at least one MOS switch bulk terminal via the buffer comprises driving the at least one MOS switch bulk terminal to substantially remove any input signal content from the source-to-bulk voltage and reverse bias at least one MOS switch source-bulk junction to control nonlinear junction capacitance associated with the at least one MOS switch.

4. A method of linearizing a MOS-based switch, the method comprising the steps of:

providing level-shifter and a MOS switch having a source, a gate, a drain and a bulk terminal; and

driving the bulk terminal via the level-shifter in response to an input signal to provide a level-shifted replica of the input signal to control the spurious dynamic range of the MOS switch, wherein the step of driving the bulk terminal via the level-shifter comprises driving the bulk terminal via a buffer having substantially unity gain and a DC voltage offset to reverse bias the source-bulk junction.

5. A method of linearizing a MOS-based switch, the method comprising the steps of:

providing level-shifter and a MOS switch having a source, a gate, a drain and a bulk terminal; and

driving the bulk terminal via the level-shifter in response to an input signal to provide a level-shifted replica of the input signal to control the spurious dynamic range of the MOS switch, wherein the step of driving the bulk terminal via the level-shifter comprises driving the bulk terminal via a buffer having substantially unity gain and a DC voltage offset to substantially remove 2nd-order effect such as nonlinear body effect (V_T(V_SB)) and nonlinear junction capacitance (C_j(V_SB)) associated with the MOS switch.

6. A method of linearizing a MOS-based switch, the method comprising the steps of:

providing level-shifter and a MOS switch having a source, a gate, a drain and a bulk terminal; and

driving the bulk terminal via the level-shifter in response to an input signal to provide a level-shifted replica of the input signal to control the spurious dynamic range of the MOS switch, wherein the step of driving the bulk terminal via the level-shifter comprises driving the bulk terminal via a buffer having substantially unit gain and a DC voltage offset to substantially remove any input signal content from the source-to-bulk voltage and reverse bias the source-bulk junction to control nonlinear junction capacitance associated with the MOS switch.

7. A MOS-based switching circuit comprising:

at least one MOS switch having a source, a gate, a drain and a bulk terminal; and

control circuitry operational to selectively drive at least one MOS switch front gate and bulk in response to an input signal and reverse bias at least one MOS switch source-bulk

junction to control at least one MOS switch source-bulk junction capacitance and linearity, wherein the MOS-based switching circuit comprises a sampling network including at least one sampling capacitor.

8. A MOS-based switching circuit comprising:

at least one MOS switch having a source, a gate, a drain and a bulk terminal; and

control circuitry operational to selectively drive at least one MOS switch front gate and bulk in response to an input signal and reverse bias at least one MOS switch source-bulk junction to control at least one MOS switch source-bulk junction capacitance and linearity, wherein the MOS-based switching circuit comprises a programmable gain amplifier (PGA) having at least one MOS switch configured to select desired input and feedback resistors to control the gain of the PGA.

9. The MOS-based switching circuit according to claim 8 wherein the PGA is an inverting amplifier.

10. The MOS-based switching circuit according to claim 8 wherein the PGA is a differencing amplifier.

11. A MOS-based switching circuit comprising:

at least one MOS switch having a source, a gate, a drain and a bulk terminal; and

control circuitry operational to selectively drive at least one MOS switch front gate and bulk in response to an input signal and reverse bias at least one MOS switch source-bulk function to control at least one MOS switch source-bulk function capacitance and linearity, wherein the control circuitry comprises a buffer having substantially unity gain and a DC voltage offset.