JPH0156385B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a signal detection circuit for extracting a minute signal superimposed on a large carrier wave or offset.

When detecting and measuring a slight change from a certain amount, that certain amount is a carrier wave or offset, and the amount of change is a signal, but this carrier wave or offset (hereinafter referred to as the offset component including the carrier wave) is To remove this, differential methods are generally used. There are various means for this, such as a bridge, but in principle it is carried out as shown in FIG.

Here, v _c1 and v _c2 are both offset components, and v _c1 is added to cancel v _c2 . Note that v _S is a signal superimposed on v _c2 . When the amplitudes of offset components v _c1 and v _c2 are matched by a gain adjuster 1 such as a potentiometer or an amplifier, and the difference is taken by a differential circuit 2, the difference output V ₀₁ is
Let the degree of matching between v _c1 and V _c2 be α (however, α = v _c2 − v _c1 / v _c1 < 1), and let the gain of gain adjuster 1 be β, then V ₀₁ = v _S + v _c2 − βv _c1 = v _S + (1+α-β)v _c1 ...(1) However, v _c2 = (1+α) v _c1 . Here, β is selected so that 1+α−β=0 (2), but if there is an error of Δβ in β, an offset component of Δβv _c1 will remain. If the allowable value of this residual offset component is n ₁ times the signal v _S , then Δβv _c1 = n ₁ v _S ...(3), and if v _S /v _c1 =, then from equation (2), Δβ/β =n ₁ /β n ₁ /(1+α) ...(4) is obtained. Therefore, even if n ₁ = 1, the accuracy required for the gain adjuster 1 is about the same as , and if the signal is very small, such as 10 ^-5 to 10 ^-6 times the carrier wave, the gain adjuster It is extremely difficult to require that much precision from 1.

Therefore, as shown in FIG.
A is once made differential, and the differential output and the offset component whose gain has been suitably adjusted by the gain adjuster 1 are once again made differential by the second differential circuit 2b. As a result, the differential output V ₀₂ is the differential output V 02 of the differential circuit 2.
If the gains of a and 2b are A _a and A _b , respectively, then V ₀₂ = A _a A _b (v _S + v _c2 − v _c1 ) − A _b βv _c1 A _a A _b {v _S + (α−β/A _a ) v _c1 } ...(5) However, v _c2 = (1 + α) v _c1 . Here, β is selected so that α−β/A _a =0 (6), but if there is an error of Δβ in β, the error is (Δβ/A _a )v _c1 . If the allowable value of this error is set to n ₂ times the signal, the following equation is obtained.

Δβv _c1 A _a = n ₂ v _S ...(7) From this, considering equation (6), we obtain Δβ/β=A _a n ₂ /β n ₂ /α ...(8). Therefore, in this case, the allowable error value in Figure 1 is set as the allowable error value n ₂ in the output.
To obtain a value equal to n ₁ , the accuracy required for the gain adjuster 1 is 1/α.

In general, a matching degree α of 10 ^-1 to 10 ^-2 can be easily obtained, so even if n ₂ = 1 and = 10 ^-5 to 10 ^-6 , the accuracy of gain adjuster 1 is 10 ^-3 to 10 ^-4 is sufficient, which is more advantageous than the case in Figure 1. Furthermore, by using the differential amplifier 2a' as the first differential circuit 2a and reversing the polarity of the reference signal to that of the offset component, the configuration can be made as shown in Fig. 3, and the second differential circuit is unnecessary. Become.

Next, specific examples of the present invention will be described. Fourth
In the figure, 3a is a fixed impedance, and 3b is a variable impedance. Further, D ₁ and D ₂ are rectifying diodes, which may be omitted if the offset component v _c is direct current. Note that in this embodiment, the differential circuit 2
A case where differential amplifiers 4a and 4b are used for a and 2b is shown. Further, in FIG. 5, a general amplifier 5 is used in place of the first-stage differential amplifier 4a in FIG. In this case, in order to subtract the offset component from the reference signal, the directions of the diodes D ₁ and D ₂ are reversed, and the offset component and the reference signal having opposite polarities are added.

Next, in Fig. 4, a controllable gain adjuster, such as a multiplying DA converter (hereinafter referred to as MDAC), is shown.
FIG. 6 shows an example in which the removal of the offset component is automated using 6. Here, 7 is a comparator and 8 is an UP/DOWN counter. In the absence of an input signal, if an unbalanced component appears at the output V ₀ , the UP/DOWN counter 8 is activated by the comparator 7.
Counts up or down according to the output level. This is repeated in synchronization with the clock CK until the output _V0 becomes zero, and the balance is maintained within the range of ±1LSB of MDAC6.

Therefore, if the UP/DOWN counter 8 is changed from the counting state to the holding state by the control signal E when there is no signal, the gain of the MDAC 6 is maintained as it is, and only the minute signal component in the input v _c can be detected. It becomes possible. Although the differential configuration shown in FIG. 3 is used in FIG. 5, the same applies to the case shown in FIG.

Furthermore, if the remaining offset component, that is, the above-mentioned ±1LSB, becomes a problem, a seventh
It is sufficient to add the circuit shown in Fig. 8 or Fig. 8. In FIG. 7, 9 is the offset component removal circuit shown in FIGS. 4 to 6. Reference numeral 10 denotes an amplifier, which is connected to a feedback circuit consisting of a capacitor 11 and an amplifier 12 via a switch 13.

In this case, the presence or absence of a signal can be detected by an appropriate means, and when the switch 13 is turned on when there is no signal, DC feedback is applied and the output becomes zero. When the switch 13 is turned off during signal measurement, the DC level is maintained, and a minute signal can be detected with the offset component set to zero. Note that it is also possible to use the differential amplifier 4b of FIG. 4 or 5 in place of the amplifier 10. In addition, when the signal does not include a DC component, as shown in FIG.
0, and the switch 13 may be provided between the input terminal of the amplifier 10 and ground.

By using the above circuit configuration, the offset component can be removed and
It becomes possible to detect signals as small as 10 ^-5 to ^{10 -7} .

Furthermore, in the embodiment of FIG. 6, the differential circuit 2
By inputting a constant reference signal V _REF instead of the output of the differential circuit 2a to the input of the differential circuit 2a, it can also be used as an automatic gain adjustment circuit. An example thereof is shown in FIG. Here, the rectifying diode D and the low-pass filter 15 may be omitted if the signal v _S is a direct current. In addition, the input side and differential circuit 2 are connected to MDAC6.
If an amplifier is connected to the V _REF input side of b, it is also possible to make the amplification degree of both the same. In this case, the comparator 7 may be directly connected instead of the differential circuit 2b.

As explained above, according to the present invention, the difference between the offset component on which the minute signal is superimposed and the first reference signal is taken, and the difference between the differential output and the second reference signal is taken. Therefore, minute signals can be detected with extremely high precision, and the effect is great.

[Brief explanation of drawings]

Figure 1 is a block diagram showing an example of a conventional circuit for detecting a minute signal superimposed on an offset component;
9 through 9 are block diagrams showing each embodiment of the present invention. 1... Gain adjuster, 2, 2a, 2b... Differential circuit, 4a, 4b... Differential amplifier.

Claims (1)

[Claims] 1. A first differential circuit to which an input signal superimposed with a minute signal and a reference signal are added, a gain adjustment circuit to which the reference signal is added, the output of the gain adjustment circuit and the first differential circuit. and a second differential circuit to which an output is applied.