Amplifiers – With periodic switching input-output
Patent
1995-10-23
1997-04-15
Mottola, Steven
Amplifiers
With periodic switching input-output
330 51, 324123R, H03F 102
Patent
active
056213506
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention concerns a circuit arrangement for amplifying a weak direct voltage signal using a direct voltage amplifier that can be periodically switched through a switching member with a periodical short-circuiter on its input terminals from the direct voltage signal to a reference signal and provides adjustment with a measurement value processor connected downstream of the direct voltage amplifier when a reference signal is applied.
A circuit means of this kind is known from the journal Review of Scientific Instruments, 59 (1988), July, No. 7, pp. 1241-1243. In this device, a direct voltage amplifier can be insulated from the voltage to be measured by a short-circuiter on the input side and connected to ground. If the direct voltage amplifier is short-circuited on its input side, its output voltage is dependent only on the offset; an output voltage of the direct voltage amplifier produced in this switching state is stored in an additional circuit unit. If, in a subsequent measuring period, the direct voltage amplifier is again at the direct voltage to be measured, the stored output voltage corresponding to the offset voltage is then subtracted from the output voltage thus obtained.
SUMMARY OF THE INVENTION
Therefore the present invention improves upon the circuit of the aforementioned kind so that the amplified direct voltage approximates, as much as possible, the weak DC voltage signal to be amplified.
In accordance with the present invention, a circuit of the aforementioned kind includes a permanently closed switching element of the switching member that is located at the input terminals of the direct voltage amplifier, in series with the periodical short-circuiter in order to amplify a signal at the output diagonal of a measuring bridge as a direct voltage signal. The common connecting point of the short-circuiter and the permanently closed switching element is connected to both a contact of another switching element of the switching member and to a connecting point of the output diagonal of the bridge. The other contact of the other switching element is connected, on the one hand, to the other switching point of the output diagonal of the bridge and, on the other hand, through an additional switching element, to the input terminal, connected to the short-circuiter, of the direct voltage amplifier.
The main advantage of the circuit according to the present invention is that, even when the direct voltage signal to be amplified is zero, but the potential of such a means generating this direct voltage signal in relation to the ground is different from that of the circuit component that generates the reference signal, no common-mode voltage is applied to the direct voltage amplifier in the case of periodic switching. The temperature-dependence of the zero point is thus avoided.
In addition, the influence of the unavoidable resistances of the individual switching elements of the switching member can be compensated for; preferred switching members include electronic switching members, whose individual switching elements have an internal resistance of 100 to 200 ohms when closed, even if manufactured by HCMOS technology. Without the inventive solution, this internal resistance would result in a non-negligible dependence of the voltage at the short-circuiter on the voltage at the output diagonal of the measuring bridge, unavoidably causing measurement errors. In the circuit according to the present invention, however, the output diagonal of the bridge no longer influences the zero state, and thus no more changes occur in the common mode.
BRIEF DESCRIPTION OF THE DRAWING
To assist in describing the present invention, the FIGURE shows an example of a circuit for amplifying a weak direct voltage signal with a short-circuiter at the input of the direct voltage amplifier in accordance with an embodiment of the present invention.
In the circuit means illustrated, a bridge 20 at a voltage U produces a direct voltage signal to be amplified U.sub.G on an output diagonal formed by c
REFERENCES:
patent: 3667041 (1972-05-01), Senour
patent: 4195266 (1980-03-01), Bingham
Mottola Steven
Siemens Aktiengesellschaft
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