JPH0440373A - Current/voltage conversion circuit - Google Patents

Abstract

PURPOSE:To form a high speed circuit without increasing a noise and to contrive an expansion of the range by increasing a loop gain with the addition of low noise operational amplifier to the circuit. CONSTITUTION:The operational amplifier(2) 3 is added in the circuit to compen sate the open loop gain of operational amplifier(1) 1. The magnifying power of several to ten-odd times is suitable for the OP(2) 3 to prevent the circuit from oscillation, and the OP(2) 3 is desirable to have lowest possible noise although unnecessary to have a low bias current. Further, by adding a circuit 4 to cancel the decrease of range caused from stray capacitances of feedback resistors (R1+R2), the effect of stray capacitances in the resistors (R1+R2) can be cancelled by a variable capacitor C1, and this is particularly effective when the value of resistors (R1+R2) is larger than 1GOMEGA.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to the measurement of minute currents, and in particular to a current-voltage conversion circuit with high-speed and low-noise characteristics. tunneling microscope) and ST
It is used in measuring instruments that apply the basic principle of M.

[Conventional technology]

Conventionally, when making picoampere (pA) level current measurements, an operational amplifier OPI with a small input bias current I (pA or less) and a feedback resistor R (with a large value) are used.
Ω) times! (Figure 5) is used for boat fishing.

(Introduced in operational amplifier textbooks, data books, etc. For example, by Takuo Okamura ("Op Amp Circuit Design JC
Q Publisher, etc.) The operating principle of this circuit is that the input '@flow 1 prisoner is
Since it does not flow into I (because of the small value of !), it all passes through the resistor R, so the output is a voltage proportional to 1, v=-1・R
It is as simple as obtaining .

] When measuring a current of pA, when R = 100MΩ, V
=O,l mV is obtained. Naturally, 1. Must be <1 pA.

The noise in this circuit is the thermal noise e generated across the resistor R.
If B (Hz) is the 0 frequency bandwidth, which is a guideline for the theoretical minimum noise and how much 7 is converted into input current, then the noise '@current i,, is ill '' 630-
B""・R"" (p Ap-p) ・・
・(11. (For example, by Takao Okamura, R Analysis Noise Mechanism JCQ Publishing) When using this circuit, due to the stray capacitance Ct existing in parallel with the resistor R, the frequency band fc of the circuit becomes fc = (2π
・R−Ct ) −′ (Hz) It is said to be determined by −121.

[Problem to be solved by the invention]

From equation (1), it is necessary to increase the resistance R in order to reduce 1. For example, at B = 10kHz, 10p
To measure the current of A, the resistance R is 100MΩ.
The above is necessary. (10 in this case is 6.39A.) Actually, in Figure 5, AD515 (manufactured by Analog Devices, Ib = 0.3pA or less) is connected to OPI, and 100MΩ is connected to R.
When we investigated the frequency characteristics using the circuit shown in Figure 6, we obtained the characteristics shown in Figure 7. In this case, the band is 4KH
When calculated as z, Ct = approximately 0.4pF. Further, the noise was about 30 pA.

However, in the same circuit, if 0PAI11 (manufactured by Burr-Brown, 1b = 2 pA or less) is used for OPI, the band is 6
It spreads to KIIz and becomes C, = 0.3pF. The fact that different C7s can be obtained while using the same R indicates that the band is not limited by equation (2), as is generally said. AD515 and 0
Comparing the performance of PAIII, there is a difference in open loop gain. It is thought that 0PAIII has a larger open loop gain, which is why the band is wider even if the same resistor R is used.

That is, there was a problem in that the performance could only be achieved below the band determined by the conventional circuit (FIG. 5). This problem has been overlooked until now.

[Means to solve the problem]

In order to solve the above-mentioned conventional problems, the present invention provides the first
As shown in the figure, the lack of gain in OPI was compensated for by another amplifier OP2.

[Effect]

According to the present invention, since the operational amplifier OPI with a small input bias ti does not have enough open loop gain for boat fishing, OF2 acts as an amplifier to compensate for the lack of gain.

〔Example〕

FIG. 2 shows an example of implementing the present invention. The magnification of OF2 is several times to a factor of several times, which is suitable for preventing oscillation of the circuit. (To prevent system instability at the second pole of OPI) OF2 used in the present invention does not need to have a low bias current, but it is desirable that the noise be as low as possible.

Fig. 3 shows an embodiment in which a circuit 4 is added to cancel the decrease in bandwidth due to the stray capacitance of the feedback resistor (R1+R2).The effect of the stray capacitance of the feedback resistor can be canceled by the variable capacitor C1. is valid when is 100 or more.

〔Effect of the invention〕

By increasing the loop gain by 11 times with OF2,
As shown in Figure 4, the frequency characteristics have been broadened by about 100 times. Furthermore, the gain peak caused by the input capacitance, as seen in the frequency characteristics of the conventional circuit (FIG. 7), was suppressed, and flat characteristics could be obtained.

According to equation (1), the noise at this time is 12.6 pA
However, the prototype @ path had a thermal noise of 30 pA, which is about twice the theoretical value.

According to the present invention, a high-speed current-voltage circuit can be constructed using easily available operational amplifiers without increasing noise.

Furthermore, the band can be expanded to the time constant determined by the +21 formula without using feedforward, which tends to oscillate, or phase compensation, which is difficult to adjust.

Various C1 compensation circuits have been proposed, but if the present invention is used in combination, it is possible to further widen the band.

[Brief explanation of drawings]

Fig. 1 shows an overview of the present invention, Fig. 2 shows an embodiment of the invention, and Fig. 3 shows an embodiment based on the circuit of Fig. 2, with a configuration that cancels the influence of stray capacitance of the feedback resistor. , FIG. 4 shows the frequency characteristics of the circuit of FIG. 2 measured using the test circuit of FIG. 6. FIG. 5 is an example of a conventional current-voltage conversion circuit. FIG. 6 shows a test circuit for a current-voltage conversion circuit. FIG. 7 shows the frequency characteristics of FIG. 5 measured using the test circuit of FIG. 6. The frequency band is about 10 times wider than that in FIG. 7, and a flat characteristic without a peak is obtained. l...Resistance bias current operational amplifier 0PIl...AC voltage generator l...Beak 2 thought to be caused by input capacitance...
・Feedback resistor R 2...Circuit under test 3...Amplifier P2 for supplementing the gain of OPl P2 Continuation of the above (spontaneous) September 19, 1990 Display of incident 1990 Patent Application No. 146959 Title of the invention Current-voltage conversion circuit correction person 1 = V, /10” Toki 6 mouth (1 other person)

Claims (2)

[Claims] (1) A current-voltage conversion circuit that uses an operational amplifier IC with a small input bias current in the input stage, and has improved frequency characteristics by adding a low-noise operational amplifier and increasing loop gain. . (2) The circuit according to item 1, which is combined with a circuit that compensates for stray capacitance of the feedback resistor.