低功率運算放大器：低功率濾波器（英文資料）

　　Introduction

　　A new family of op amps features industry leading speed versus supply current. The LTC6261/LTC6262/ LTC6263 family （single， dual， quad） provides 30MHz at a low 240μA supply current， with 400μV maximum offset voltage and rail-to-rail input and output. In combination with 1.8V to 5.25V supply， these op amps enable applications requiring uncompromised performance with low power and low voltage.

　　Active Filters

　　This new set of op amps with high MHz-to-mA ratios provides an opportunity to look at filter circuits in a new light. The two filter examples discussed here display performance previously unachievable with such low power.

　　Second Order Bessel Filter Ample bandwidth and low supply current enable deployment of active filters in portable and other low power applications. For instance， a second order Bessel filter， shown in Figure 1， provides a clean transient response at the expense of a less steep roll-off in the frequency domain.

　　Measured supply current consumption is about 230μA， although data sheet supply maximum values suggest that the consumption across production and temperature may be slightly higher. The values of resistors chosen minimize consumption at the expense of in-band noise.

　　If VREF is derived from a high impedance resistor divider， then a large capacitor is required to ensure that the reference voltage is solid down to very low frequencies. The reference at the positive op amp input must be a good “AC ground” at all frequencies when using this inverting amplifier configuration.

　　The frequency response shows an expected roll-off of two poles along with a gentle droop near the 3dB point; the transient response is very clean.