CMOS track and hold amplifier

Details CMOS track and hold amplifier CMOS track and hold amplifier

2000-03-08

2001-10-30

Lam, Tuan T. (Department: 2816)

Amplifiers

With periodic switching input-output

C327S095000, C327S096000

Reexamination Certificate

active

06310515

ABSTRACT:

RELATED APPLICATIONS
This Application claims priority under 35 U.S.C. §119 from Norway Patent Application No. 1999 1320, filed Mar. 18, 1999.
1. Technical Field
The present invention is related to a switched Capacitor Track and Hold Amplifier having at least one signal input, at least one clock input and an output.
2. Background
CMOS THAs are generally implemented as switched capacitor (SC) circuits. This type of THA is commonly used in the industry and substantial effort has been put into the optimization of the architecture and switch configuration to reduce the effect of non-idealities. A typical implementation of a switched capacitor THA is shown in FIG.
1
.
The circuit operates in two phases. In phase
1
, the sampling phase, the switches labelled &phgr;
1
are closed. In phase
2
, the hold phase, the switches labelled &phgr;
2
are closed. The phases are non-overlapping so that the &phgr;
1
switches are opened a short instant before the &phgr;
2
switches are closed. In phase
1
the input signal is charging the sampling capacitors C
1
and C
2
. At the end of phase
1
switches S
3
and S
4
opens prior to S
1
and S
2
. The sampling instant is therefore controlled only by the switches S
3
and S
4
. When all switches are open, the input voltage is stored on the sampling capacitors. In phase
2
the switches S
5
and S
6
are closed, copying the sampled voltage to the THA output. The gain of the operational transconductance amplifier (OTA) must be high in order to maintain a high accuracy.
The switches in
FIG. 1
are implemented as MOS transistors. A symbol of a N-type MOS transistor (NMOS) is shown in FIG.
2
.
When the MOS transistor is operated as a switch, it acts like a voltage controlled resistance. The device is symmetrical so that source may act like drain and drain may act like source determined by the voltages at the different nodes. A NMOS device is switched on when the voltage between gate and source (V
GS
) goes higher than the threshold voltage (V
T
)of the device. A PMOS device is switched on when V
GS
goes more negative than the V
T
(negative for PMOS) of the device.
The switch ON-resistance is determined by V
GS
−V
T
. In addition V
T
is changed by the voltage between bulk and source. The switches in a SC circuit is normally controlled by CMOS digital signals. The gates are therefore either connected to ground or to the supply voltage. As a result of this, V
GS
−V
T
and therefore also the switch ON-resistance for some of the switches, is dependent on the voltage of the signal nodes in the circuit.
Switch S
5
and S
6
in
FIG. 1
is not sensitive to the switch ON-resistance since the output signal settles to a constant voltage in the hold phase and the current through the switch goes to zero. Switch S
3
and S
4
are connected with source to ground and V
GS
−V
T
is therefore kept constant. The input switches have their source connected to the input signal and V
GS
−V
T
is directly controlled by the input voltage. As a result of this, the switch ON-resistance is modulated by the input signal and haimonic distortion results when a high frequency input signal is applied to the circuit.
This problem is known from prior air, and several principles have been proposed to enhance the performance of the input switch.


REFERENCES:
patent: 5572154 (1996-11-01), Rakers et al.
patent: 5638020 (1997-06-01), Koifman et al.
patent: 5644257 (1997-07-01), Kerth et al.

Affiliated with

Also associated with

Rating

CMOS track and hold amplifier does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with CMOS track and hold amplifier, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and CMOS track and hold amplifier will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2611759