Operational Experience of Op Amp OPA690

Table 1 Relationship Between Gain and Bandwidth (with ±5V Power Supply)

±2.5~±5V Dual Supply or +5~+12V Single Supply

Output voltage range ±4V

Output current 190mA

Disable current 100uA

Figure 1 pin layout

It is the same as the general opamp pin layout. The only difference is that the 8-pin port is a low power mode control port. When this pin is left floating or high, it is in normal operation. If it is set low, the supply current will drop to 100uA. The chip stops working and is in power saving mode.

Figure 2 reverse amplifier circuit

In actual application, the circuit is connected according to the above figure. The figure above shows the magnification of 2 times and the requirement of 10 times amplification. You need to select RF as 1KΩ (200Ω~1.5KΩ can be satisfied) and RG=100Ω. If impedance matching is needed, it will be The RM is connected in parallel to RG to adjust the input impedance. At the same time, after the three resistors are determined, the value of RB is determined again so that the DC resistances of the non-inverting input and the inverting input are equal to each other to determine the value of RB. This causes the input DC bias current to be reduced to a minimum. The 0.1uF capacitor acts as a voltage coupling to reduce waveform distortion caused by the second harmonic output.

When the input signal frequency is 100KHz, the signal can be amplified by 10 times, and the frequency is gradually increased. When the signal is adjusted to 20MHz over 5M, the amplified output signal gradually becomes smaller, and the output signal becomes distorted when it approaches 20MHz. . Try to increase the frequency when the distortion is even more severe.

Figure 3 Small Signal Input Frequency Response Curve

Compared with the above figure, we can see that, in the case of small signal input, when the gain is 10, the input signal frequency is greater than 4MHz, the output begins to decline, more than 10MHz decline more powerful, which concluded that in the amplification When the multiple ≥10, the OPA690's high frequency performance is not good.

Figure 4 Large Signal Input Frequency Response Curve

When the input signal is a large signal, the frequency of the input signal can be significantly improved when the amplification factor is 10, and the high frequency performance is slightly better when the large signal is input.

For amplification applications, Rf is best recommended in the data sheet, especially when doubling and five times magnification.

When the OPA690 is used as an emitter follower, the feedback loop cannot be directly short-circuited, and the 25Ω resistor must be connected.

OPA690 is best not to use the most emissive device, easy to produce self-excitation

When G "1", Rf value range is limited, 200Ω "Rf" 1.5KΩ

The empirical value of Rf||Rg is Rf||Rg “300Ω

Extra supplement:

Fig. 5 High-frequency Butterworth filter

The above figure shows a second-order band-pass filter with a center frequency of 5 MHz. It can also amplify the signal by a factor of three.

In addition, the active filter is generally only able to do about 500K, so do not worry about the analog bandwidth of the op amp, as long as the op amp with a good slew rate is selected. As for the impact of analog bandwidth on the signal, only OPA690, which can disable the bandwidth, can limit the bandwidth of the signal. The others are less strict. A 5M filter, through a 4M square wave, will not produce large waveform distortion, which is not related to the cutoff frequency of the filter, and is related to the group delay of the filter. So usually we say that the filter cut-off frequency refers to the single-frequency signal, which is the sine wave, or the main frequency of the wide-spectrum signal. Don't worry about the effect on harmonics.