JPS61198911A - Switched capacitor filter - Google Patents

Abstract

PURPOSE:To process a signal having various signal speeds at a low output noise by connecting a circuit having a switch and a capacitor operated at a frequency MfS (1<M) to an input terminal or an output terminal of an operational amplifier. CONSTITUTION:The circuit having the switches 101, 102 and the capacitors 103, 104 operated at a frequency MfS (1<M) is connected to an input terminal or an output terminal of the operational amplifier 3. The switches 101, 102 are driven by clocks phi10 and phi20. M-set of sample values are added to the capacitor 103 at one period of the clocks phi1, phi2 with respect to noise component. M-set of noise samples are added also to the capacitor 104 and averaged. Thus, the signal having various signal speeds is processed with low output noise.

Description

[Detailed description of the invention] The present invention relates to switched capacitor filters.

The configuration of a conventional switched capacitor filter (hereinafter abbreviated as SCF) is shown in FIG. 1(a). In the figures, reference numerals 1 and 12 are switches, respectively in Figure 1 ←
) is closed when the clocks φ and φ2 are at high level, and opened when the clocks are at low level. In this way, the SCF operates by periodically repeating opening and closing of the switch. By opening and closing this switch, the stain pressure (charge) is sampled and stored in each capacitor. Therefore, the SCF constitutes a sample-based filter that processes the sampled signal. 5CFK: The operational amplifier 3 used generates noise. This is mainly l
/f noise and white noise. This white noise extends to a high frequency band, and there is a problem in that it is reflected back to the fundamental frequency band and amplified by sampling inside the SCF. This situation is shown in FIG. Figure (a) shows the relationship between the output noise f of the operational amplifier and the electric waste vector P,
The figure Φ) shows the electric scum vector when sampled at the sampling frequency f. Letting the bandwidth of the output noise be f., it becomes KFif@/fs in FIG. 2(b). In other words, the noise power is l Oflog (f, /f,
) is amplified by dB. On the other hand, the on-resistance of an actual switch in the closed state is not zero, but is usually lO~30 (KΩ).
That's about it. This on-resistance and the capacitor form a first-order low-pass filter (LPF), and fc
will decrease isometrically, and the value of K will also decrease.

Therefore, in order to reduce the noise in the filter output, a method can be considered in which the on-resistance of the switch is increased and f itself is decreased. However, for example, when processing signals in various bands, these parameters need to be selected for the highest band signals, and in that case, the sampling frequency f, should be selected for the lowest band signals. When processing is performed with a lower value, =f, /f naturally becomes larger.

An object of the present invention is to provide an SCF that can process signals having various signal speeds with low output noise.

The SCF of the present invention is composed of a plurality of switches, capacitors, and operational amplifiers, and has a sampling frequency f.

In a switched capacitor filter that operates at
A circuit having a switch and a capacitor operating at (L<M) is connected.

Next, the present invention will be explained in detail with reference to the drawings.

FIG. 3 is a circuit diagram showing one embodiment of the present invention, and FIG. 4(a) shows high-speed integrators 201 and 20 used in this embodiment.
FIG. 2 is a circuit diagram showing details of No. 2;

In FIG. 4(a), switches 101 and 102
is driven by clocks φ1o and φ2o shown in FIG. Capacitor 103 has 1 of φ1 and φ2 for noise components.
M sample values are added every 2 cycles.

In addition, a capacitor 10 connected to the output side of this integrator
M noise sample values are also added to 4 and averaged.

In FIG. 3, noise components are added to capacitors 22, 26' and 27 while switch 11 is closed.

Noise components are also added to the integrating capacitor 103 until its output is sampled. Therefore, the noise component contained in the signal sampled by the clocks φ1 and φ2 becomes the sum of M or M72 sample values, and uncorrelated noise is reduced by averaging. FIG. 5 is an equivalent circuit of 8CF shown in FIG. What can be said qualitatively from this is that high-frequency noise components are suppressed by the primary LPF having a resistor and a capacitor, and the frequency f.

This means that the aliasing component when sampling is reduced. In this case, the equivalent resistance 1 is composed of an SC circuit,
It is possible to achieve higher precision than actual resistance.

As described above, the present invention has the effect of achieving a reduction in output noise of 80F.

[Brief explanation of the drawing]

Figure 1 (a) and (a) are a circuit diagram showing a conventional SCF and a waveform diagram showing a clock signal, respectively.
) and (b) are waveform diagrams illustrating clock signals and for explaining the drawbacks of the conventional SCF, and FIG. 5 is a circuit diagram showing an equivalent circuit of FIG. 3. In the figure, l...Equivalent resistance, 3...Operation amplifier, 4...Input terminal, 5...Output terminal, 11°12, 101.102... ...Switch, 103,104゜22.23,24,26,27
．． 28... Capacitor, 201, 202...
...integrator. is represented by patent attorney Susumu Uchihara (.

Claims (1)

[Claims] In a switched capacitor filter that is composed of a plurality of switches, capacitors, and operational amplifiers and operates at a sampling frequency f_s, a frequency Mf_s (1<M) is applied to at least one of the input terminal or output terminal of the operational amplifier.
1. A switched capacitor filter, characterized in that a circuit having a switch and a capacitor that operates is connected to each other.