Patent
1996-05-15
1999-09-28
Teska, Kevin J.
G06F 9455
Patent
active
059601833
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a method of and apparatus for facilitating the accurate and continuously tunable processing of analogue electrical signals in a fully integratable way, without the need for post-manufacture trimming.
There are two basic ways in which analogue electrical signals can be processed, that is in the continuous or discrete time domains. In practical terms both methods have their pros and cons.
Continuous time filters, such as the transconductance-capacitor technique achieving very good inherent precision without resorting to a tuning scheme or trimming is not possible. This is because the accuracy of continuous time filters depends on the absolute value of components or the product or ratio of absolute values of components of differing types. Using standard integrated circuit processing technology it is not possible to reliably manufacture components whose absolute values are very accurately controlled without the use of post-manufacturing trimming. Furthermore, the absolute values of components is subject to drift with time, temperature and other physical stresses, thus a signal processor relying on absolute values will suffer from drift. Complex single or two loop on-chip tuning schemes may be employed to alleviate this problem.
Discrete time analogue signal processors, such a switched-capacitors or switched-currents, overcome the accuracy difficulties of continuous time processors by relying on the ratio of capacitor values or transistor sizes respectively. In integrated circuit form, it is possible to match the values of components of the same type accurately and thus realise very accurate discrete time analogue signal processors. Furthermore, the effect of component drift is much less severe because all the matched components will experience the same drift mechanisms. Although discrete time analogue signal processors exhibit good accuracy they have two very significant limitations. The first is that the signal bandwidth is limited to less than half the clock frequency, which is itself limited by the need to ensure good settling performance. This means that discrete time analogue signal processors are restricted to lower signal frequencies than their continuous counterparts for a given technology. The second disadvantage is that it is not possible to realise continuously tunable discrete time analogue signal processors and achieving even a degree of tunability requires cumbersome circuitry. The reason for this is that a ratio of components, capacitors in the case of switched-capacitors, determines the cut-off frequency and to tune this requires the switching in or out of capacitors. Thus to achieve good tunability in switched capacitors requires a large bank of capacitors occupying considerable area.
The present invention seeks to provide inherently accurate and continuously tunable analogue signal processors which can be manufactured using a standard digital integrated circuit process.
Accordingly the present invention provides a signal processing circuit comprising a tunable impedance which is made up of a tunable continuous time transconductor or transresistor and a discrete time current or voltage processor connected between its input and output so as to feed its output current or voltage back to its input. Thus the circuit presents an input impedance which is equal to the inverse of the product of the transfer functions of the transconductor and current processor (or transresistor and voltage processor). The following discussion is given generally in terms of transconductors and current processors but it will be appreciated that the same principles apply in respect of transresistors and voltage processors.
If the discrete time current processor is a scaler the overall input impedance mimics that of a resistor, if it is an integrator it mimics an inductor and if it is a differentiator it mimics a capacitor. Combination of switched-transconductance circuits, containing the above described discrete time current processors (or other similar current processors) perform a wide variety
REFERENCES:
patent: 4035688 (1977-07-01), Mourier
patent: 5189321 (1993-02-01), Seevinck
patent: 5559470 (1996-09-01), Laber et al.
Manfredi et al., Realization of some analog functions with switchable feedback loops, Milano, pp. 328-333, Apr. 29, 1982.
Khoury et al., Synthesis of arbitrary rational transfer functions in S using uniform distributed RC active circuits, IEEE, pp. 464-472, Apr. 1990 .
Battersby, deceased Nicolas
Toumazou Christofer
Imperial College of Science Technology & Medicine
Loppnow Matthew
Teska Kevin J.
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