LU101199B1

LU101199B1 - A readout circuit based on chopping modulation and correlated double sampling

Info

Publication number: LU101199B1
Application number: LU101199A
Authority: LU
Inventors: Jianguo Ma; Shaohua Zhou
Original assignee: Univ Tianjin
Priority date: 2018-09-20
Filing date: 2019-04-30
Publication date: 2019-08-26
Also published as: CN109450384A

Abstract

The present invention discloses a readout circuit based on chopping modulation and correlated double sampling, comprising a chopper, wherein an output terminal of the chopper is sequentially connected in series with a preamplifier, a GmC low-pass filter, a correlated double sampling unit and a low pass filter; a signal is modulated to a high frequency by the chopper, and further amplified by the preamplifier, then enters the GmC low-pass filter to output a differential sinusoidal signal at a chopping frequency, passes through the correlated double sampling unit so that the flicker noise introduced by preamplifier is cancelled while the offset noise introduced by the chopper is also eliminated, and finally passes through the low pass filter to be filtered out. The present invention provides a sufficient gain to amplify a voltage signal output by a sensor, and can suppress noise, mainly suppressing the flicker noise.

Description

A READOUT CIRCUIT BASED ON CHOPPING MODULATION AND CORRELATED DOUBLE SAMPLING

Technical field

The present invention relates to the field of sensors, and more particularly to a readout circuit based on chopping modulation and correlated double sampling.

Technical background

The terahertz band is located between the millimeter band and the infrared band, and the electromagnetic band with a frequency range of 300 GHz to 3 THz is defined as the terahertz band. Since the energy of terahertz photons is very low, and the energy of 1 THz photons is only about 4 millielectron volts, it is not easy to destroy substances to be detected, which makes the terahertz band have certain advantages in terms of imaging over infrared imaging.

The mainstream technology adopted by terahertz focal plane detectors abroad is based on microbolometers, and their materials include VOx and polysilicon. In 2006, the US Jet Propulsion Laboratory successfully developed a terahertz imaging system capable of high-resolution ranging [1], When the distance target is 4 meters, the resolution is about 2 cm. In 2010, the SLR laboratory at the University of Massachusetts State University in US conducted an inverse synthetic aperture radar imaging experiment, which has been able to obtain a high-resolution scale-down model.

Regardless of the form of the terahertz detector, its output signal is extremely small, resulting in a low signal-to-noise ratio of the output signal. Therefore, an additional readout circuit is required to amplify the signal while the noise can be effectively suppressed. Base on the current need for this readout circuit characteristic, a readout circuit based on a combination of chopping modulation, selective amplifiers and correlated double sampling is proposed.

[References] [1] Reck T, Sites J, Jung C et al. Array Technology for Terahertz Imaging Passive and Active Millimeter-Wave Imaging XV[C]. Proc. SPIE. Baltimore, 2012.

[2] Cezary Kolacinski, Dariusz Obrebski. The integrated selective readout amplifier for NMOS THz detectors[C]. Mixed Design of integrated Circuits and Systems, 2014: 272-277.

[3] C.C.Enz, GC.Temes. Circuit techniques for reducing the effects of op-amp imperfections: autozeroing, correlated double[J], Proceedings of the IEEE, 1996, 84(11): 1584-1614.

[4] Massimiliano Belloni, Edoardo Bonizzoni, Andrea Fomasari, Franco Maloberti. A Micropower Chopper—CDS Operational Amplifier[J]. IEEE Journal of Solid-State Circuits, 2010, 45(12): 2521-2529.

Summary of the invention

An object of the present invention is to overcome the deficiencies in the prior art and provide a readout circuit based on chopping modulation and correlated double sampling, which provides a sufficient gain to amplify a voltage signal output by a sensor, and can suppress noise, mainly suppressing the flicker noise.

The object of the present invention is achieved by the following technical solutions. A readout circuit based on chopping modulation and correlated double sampling of the present invention comprises a chopper, wherein an output terminal of the chopper is sequentially connected in series with a preamplifier, a GmC low-pass filter, a correlated double sampling unit and a low pass filter; a signal is modulated to a high frequency by the chopper, and further amplified by the preamplifier, then enters the GmC low-pass filter to output a differential sinusoidal signal at a chopping frequency, passes through the correlated double sampling unit so that the flicker noise introduced by preamplifier is cancelled while the offset noise introduced by the chopper is also eliminated, and finally passes through the low pass filter to be filtered out.

The chopper has three input terminals, one being connected to a signal source, one being grounded, and one being connected to a clock signal.

Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

Compared with a simple low noise readout circuit having chopper modulation plus a filter, the present invention can reduce the offset noise introduced by the chopper to a greater extent due to introducing a correlated double sampling circuit as a demodulation unit, thereby further reducing the corner frequency of the flicker noise and improving the signal-to-noise ratio of the output of the readout circuit.

Brief description of the drawings

Fig. 1 is a schematic diagram of a basic chopper modulation-based readout circuit;

Fig. 2 is a schematic diagram of a readout circuit based on chopping modulation and correlated double sampling of the present invention.

Reference signs: Yin: signal source; GND: ground; £*οΡ: clock signal.

Detailed description of the embodiments

In order to more clearly explain the technical solutions of the present invention, the present invention will be further described below in conjunction with the accompanying drawings.

The present invention is an improvement of a readout circuit based on a chopping modulation method. A simple chopper modulation-based readout circuit is as shown in Figure 1. A DC signal is modulated to a high frequency by a first stage chopper 1, and further amplified by an AC amplifier 2 while the noise of the AC amplifier is introduced, and finally is demodulated back to a low frequency by a second stage chopper 3. However, the noise introduced by the AC amplifier is only subjected to the modulation of the second-stage chopper, so the noise is modulated to a high frequency, so that the noise and signal are separated, and finally a useful signal can be filtered out through a low pass filter. Some teams have improved on this basis by connecting a low pass filter behind the AC amplifier to limit the bandwidth of the AC amplifier [2], which can further reduce the noise generated by the readout circuit. It is because if the bandwidth of a τ amplifier is infinite, then the offset noise generated by this chopping modulation method is [3]:

(1)

However, if a bandpass filter is connected after the AC amplifier and the stopband frequency is limited to about twice the chopping modulation frequency, the generated offset noise is:

(2)

Since z<772, the offset noise generated after limiting the bandwidth can be reduced.

However, since the chopper itself introduces the offset noise, the present invention on this basis replaces the second-stage chopper with a correlated double-sampling circuit to perform signal demodulation, which can overcome the offset noise introduced by the chopper, thereby further reducing the corner frequency of the flicker noise.

As shown in Fig. 2, a readout circuit based on chopping modulation and correlated double sampling of the present invention comprises a chopper 4, wherein an output terminal of the chopper is sequentially connected in series with a preamplifier 5, a GmC low-pass filter 6, a correlated double sampling unit 7 and a low pass filter 8. The chopper has three input terminals, one being connected to a signal source Vin, one being grounded GND, and one being connected to a clock signal fchop- A positive output terminal of the chopper is connected to a positive input terminal of the preamplifier, an inverting output terminal of the chopper is connected to an inverting input terminal of the preamplifier, the positive output terminal of the preamplifier is connected to an positive input terminal of the GmC low-pass filter, an inverting output terminal of the preamplifier is connected to an inverting input terminal of the GmC low-pass filter, and it is the same analogy for the related double-sampling unit and the low pass filter. For all of them, a positive output terminal is connected to a positive input terminal and an inverting output terminal is connected to an inverting input terminal.

When a DC signal outputted by a detector enters the readout circuit based on chopping modulation and correlated double sampling of the present invention, it is modulated to a high frequency by the chopper and then is amplified by the preamplifier. Since the cutoff frequency of the GmC low-pass filter is set at about twice the chopping frequency of the chopper, when the signal enters the GmC low-pass filter after it is amplified by the preamplifier, a differential sinusoidal signal with the chopping frequency is output, and passes through the correlated double-sampling unit. Since the flicker noise at a low frequency has strong time correlation, the correlated double sampling unit can cancel the flicker noise introduced by the preamplifier while the offset noise introduced by the chopper is also eliminated. Finally, the signal is filtered out through a low pass filter. Therefore, the structure can better suppress the flicker noise than the basic chopper modulation-based readout circuit.

Although the functions and working processes of the present invention have been described above with reference to the accompanying drawings, the present invention is not limited to the specific functions and working processes described above, and the specific embodiments given above are merely illustrative and not restrictive. Many forms may be made by those skilled in the art under the inspiration of the present invention without departing from the spirit and scope of the invention, and these are all within the protection of the present invention.

Claims

A chopper modulation-correlated double sampling read-out circuit comprising a chopper, characterized in that: an output terminal of the chopper is serially connected in series with a pre-amplifier, a GmC low-pass filter, a correlated double sampling unit and a low-pass filter; wherein a signal is modulated by the chopper to a high frequency and further amplified by the preamplifier, and then enters the GmC low pass filter to output a differential sinusoidal signal having a clock frequency which then passes through the correlated double sampling unit so that that through the preamplifier caused flicker noise is canceled, while the offset noise introduced by the chopper is also eliminated, and it eventually passes through the low-pass filter to filter it out.

2. Read-out circuit based on a chopping modulation and a correlated double sampling according to claim 1, characterized in that the chopper has three input terminals, one of which is connected to a signal source, one is connected to ground and one is connected to a clock signal. Tk TTaktsignal is connected.