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A simple measurement of the op amp

In Electronic Infomation Category: A | on May 04,2011

Op amp is a differential input, single-ended output high-gain amplifier, commonly used in high-precision analog circuits, it must accurately measure its performance. However, in the open-loop measurement, the open-loop gain may be as high as 10 7 or higher, and LTC1159CS-3.3 datasheet and pick up the stray current or Seebeck (thermocouple) effects may be produced at the amplifier input is very small voltage, so the error will be difficult to avoid.

Through the use of servo-loop, we can greatly simplify the measurement process, forcing the amplifier input zero, making the amplifier under test to measure their own errors. Figure 1 shows a multi-functional circuits using the principle, which uses an auxiliary operational amplifier as integrator, to create a highly stable DC open-loop gain loop. Switch for the implementation of the various tests described below provide a convenience.


Figure 1. Basic operational amplifier measuring circuit

The circuit shown in Figure 1 most of the measurement error can be reduced to a minimum, support a large number of DC and LTC1159CS-3.3 price and a small amount of accurate measurement of AC parameters. Additional "auxiliary" do not need to have the op-amp op amp under test better than the performance, the DC open loop gain is best to reach 106 or higher. If the device under test (DUT) offset voltage may be more than a few mV, then the auxiliary op amp should be used 15 V power supply (if the DUTs input offset voltage may exceed 10 mV, the need to reduce the resistance of 99.9 k resistor R3 .)

DUT supply voltage + V and-V amplitude equal, opposite polarity. The total power supply of course is 2 V. Symmetric power of the circuit, even though the "single supply" op amp is also true, because the system in the middle of the ground to supply voltage reference.

As the integrator when the auxiliary amplifier is configured for open loop DC (maximum gain), but the input resistance and LTC1159CS-3.3 suppliers and feedback capacitance to its bandwidth is limited to a few Hz. This means that, DUT output DC voltage is the highest gain auxiliary amplifier to amplify, and through a 1000:1 attenuator applied to the noninverting input of the DUT. Negative feedback will DUT output driver to ground potential. (In fact, the actual voltage is to assist the amplifier offset voltage, more precisely, the auxiliary amplifier offset voltage plus the bias current induced in the 100 k resistor voltage drop, but it is very close to ground potential, so it does not matter especially considering this point during the measurement of voltage changes is unlikely to exceed a few mV).

Test point TP1 on the DUT voltage is applied to the calibration input voltage (with the error rate on the equivalent) 1,000 times, about tens of mV or more, it can be fairly easily measured.

Ideal op amp offset voltage (V os ) is 0, that is, when the two input terminals connected together and to keep the middle of the supply voltage, the output voltage is the same for the middle of the supply voltage. Real op-amp is a few microvolts to several millivolts offset voltage ranging and must therefore be within this range of input voltage is applied, so that the output potential in the middle.

Figure 2 shows the most basic test - the offset voltage measurement configuration. When the voltage on TP1 offset voltage for the DUT 1000 times, DUT output voltage is at ground potential.


Figure 2. Offset voltage measured

Ideal op amp has infinite input impedance, no current flows into its input. But in reality, there will be a small amount of "bias" current into the inverting and inverting input (respectively, Ib-and Ib +), they will be in high impedance circuits caused a significant offset voltage. According to the different types of operational amplifiers, this bias current may be several fA (1 fA = 10 -15 A, every few microseconds flow through an electronic) to several nA; in some super- Fast op amp, and even reach 1 - 2 A. Figure 3 shows how to measure these currents.


Figure 3. Offset and bias current measurement

The circuit in Figure 2, the offset voltage of the circuit is basically the same, but the addition of two DUT input series resistor R6 and R7. These resistors can be shorted through switches S1 and S2. When two switches are closed, the circuit in Figure 2 are identical. When S1 is off, the inverting input bias current into the Rs, the voltage difference increased to offset voltage. TP1 by measuring the voltage change (= 1000 I b- R s ), can be calculated I b-. Similarly, when S1 and S2 closed off, you can measure I b + . If S1 and S2 are first measured at TP1 voltage closed, S1 and S2 are then broken again when the voltage measured TP1, through the voltage changes can be measured out the "input offset current" Ios, that I b + and I b difference. R6 and R7 to measure the resistance depends on the current size.

If the value of Ib is about 5 pA, will use large resistor, the circuit will be very difficult to use, you may need to use other techniques, involving Ib to the low leakage capacitor (used in place of Rs) charge rate. When S1 and S2 closed

time, Ios will flow into 100 resistor, resulting in V os error, but usually can be ignored in the calculation of it, unless the Ios is large enough, the resulting error is greater than the measured V os 1%.

Op amp open-loop DC gain can be very high, more than 107, the gain is not rare, but the gain of 250,000 to 2,000,000 is more common. DC Gain measurement method is switched through S6 and the DUT output 1 V reference voltage between the R5, forcing the DUTs output to change a certain amount (Figure 4, for 1 V, but if the device is large enough power supply, you can provisions of 10 V). If R5 is +1 V, the input to the auxiliary amplifier to maintain near constant at 0, DUT output must be changed -1 V.


Figure 4. DC Gain measurement

TP1 1000:1 attenuation of voltage change and enter the DUT, causes the output to change 1 V, which is easy to calculate the gain (= 1000 1 V/TP1).

To measure the open-loop AC gain, you need to inject a DUT input the desired frequency of small ac signal, and measure the corresponding output signals (Figure 5 TP2). Completed, the auxiliary amplifier output to the DUT, the average DC level remained stable.


Figure 5. AC gain measurements

Figure 5, the AC signal through the attenuator 10,000:1 input applied to the DUT. The DC open loop gain could be close to the value of low frequency measurement, you must use such a large attenuation. (For example, when the frequency of gain of 1,000,000, 1 V rms signal will be 100 V applied to the amplifier input, the amplifier is trying to provide 100 V rms output, leading to the amplifier saturation.) Thus, the frequency of AC measurement is generally a few hundred Hz to the open-loop gain to 1 when the frequency; in the low-frequency gain of the data need to be very careful to use lower input range of measurement. A simple attenuator shown only in frequencies below 100 kHz, even if carefully handled can not stray over the frequency. If related to higher frequencies, you need to use more complex circuits.

Op amp common-mode rejection ratio (CMRR) refers to the common-mode offset voltage change as a result of changing the applied common mode voltage change ratio. In DC, it is generally between 80 dB to 120 dB, but at high frequencies will be reduced.

Test circuit is suitable for measuring CMRR (Figure 6). It is not the common-mode voltage applied to the DUT input, so as not to undermine the effect of measuring low level, but to change the supply voltage (relative to input the same direction, that is, the direction of common mode), the rest of the circuit remain unchanged.


Figure 6. Dc CMRR measurement

The circuit shown in Figure 6, measured in the TP1 offset voltage, power supply voltage of V (in this case, +2.5 V and -2.5 V), and once again move the two supply voltage +1 V (to + 3.5 V and -1.5 V). Offset voltage change corresponding to 1 V common mode voltage change, so the DC offset voltage and CMRR as the ratio of 1 V.

CMRR to measure the offset voltage relative to the common-mode voltage changes, the total power supply voltage remains unchanged. Power supply rejection ratio (PSRR) on the contrary, it refers to the offset voltage power supply changes and changes in the total ratio between the common mode voltage to maintain constant power supply voltage (Figure 7).


Figure 7. DC PSRR measurement

Circuit used exactly the same except that the total power supply voltage change, while the common mode level remains the same. In this case, power supply voltage from +2.5 V and -2.5 V to switch to the +3 V and -3 V, the total supply voltage changes from 5 V to 6 V. Common-mode voltage remains between the supply voltage. Calculation are the same (1000 TP1 / 1 V). To measure the exchange

CMRR and PSRR, the voltage needed to modulate the supply voltage, as shown in Figure 8 and Figure 9. DUT continues to work in the DC open-loop, but the exact exchange gain of negative feedback from the decision (the figure is 100 times.)


Figure 8. AC CMRR measurement To measure the exchange

CMRR, using the peak amplitude of 1 V AC voltage modulation DUT positive and negative power supply. Modulation of the two-phase power supply, so the actual supply voltage stable DC voltage, but the common-mode voltage is 2V peak to peak sine wave, resulting DUT output includes an AC voltage at TP2 measured.

If TP2 x V AC voltage with peak amplitude (2x V peak to peak), the equivalent to the DUT input (ie, the exchange gain of 100 times before) and CMRR is x/100 V, and the value of CMRR 1 V peak ratio.


Figure 9. AC PSRR measurement

AC PSRR measurement method is the AC voltage is applied to a 180 phase difference between the positive and negative power supply voltage to the amplitude modulation (in this case is also a 1 V peak, 2 V peak to peak), while the common-mode voltage remains stable DC voltage. Parameters calculated with the previous calculation method is very similar.

Summary

, Of course, there are many other parameters of the op amp may be measured, but there are many other ways to measure the above parameters, but as this article shows, the most basic use of DC and AC parameters can be easy to build, easy to understand, mm No question the basic circuitry of simple and reliable measurements.

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