對高阻抗緩沖放大器的輸入實現(xiàn)ESD保護

　二極管和保護裝置抑制瞬時高壓靜電放電。

　　在某些測量應用中，如pH值（酸度）和生物電勢需要高阻抗緩沖放大器。雖然有幾個半導體制造商提供具有低偏置和輸入偏置電流特點的放大器芯片，配上傳感器電纜可能會因為ESD（靜電放電）對放大電路造成損害。圖1顯示一種不令人滿意的方法實現(xiàn)ESD保護。電阻R₁抑制ESD的放電電流，而二極管D_1A 和D_1B鉗住放大器IC₁的輸入到供電電源軌。不幸的是，當并聯(lián)pH值傳感器的為400MΩ輸入阻抗時，即使采用低漏電流二極管，如Fairchild公司的MMBD-1503A，仍引入很大的偏置電壓。

　　圖2中的電路提供另一個方法。Analog Devices公司的低輸入偏置，低偏置電流的AD8603放大器IC₁，作為單位增益輸入緩沖器。對于任何正常輸入，電路的輸出電壓V_OUT 等于其輸入電壓V_IN。因此，電壓經(jīng)過ESD保護二極管D_1A 和D_1B接近0V，二極管漏電流均不影響傳感器輸出信號。依靠ESD適用電路輸入連接器的極性，其高電壓放電通過二極管D_1A 或D_1B進入供電正向或負向電源。電容C₁充當中間“電荷存儲器”，放慢ESD的上升速率，保護IC₁的輸出過程，從鎖存到二極管D_1A 或D_1B開始分離ESD瞬流到正向或負向電源。實際上，C₁補償D₁的寄生電容。電阻R₃允許IC₁來驅(qū)動電容負載，使C₁不會進入振蕩狀態(tài)。

　　在ESD事件中，D₁和D₂可以進行傳導，但通過兩個前置偏移二極管壓降，V_IN端的電壓超出供電電源電壓。電阻R₁和R₂限制放大器輸入電流在制造商建議的最大5mA之下。

　　封裝電路時，需要特別注意電路板的布局。電路板介電性能的缺陷會給寄生漏電流提供路徑。在電路板的兩面敷銅形成保護環(huán)，可以在電路高阻抗節(jié)點周圍轉(zhuǎn)移漏電流（圖3）。

　　英文原文：

　　High-impedance buffer amplifier's input includes ESD protection

　　Diodes and guard traces suppress high-voltage static-discharge transients.

　　Eugene Palatnik, Waukesha, WI; Edited by Brad Thompson and Fran Granville -- EDN, 9/28/2006

　　Certain measurement applications, such as for pH (acidity) and bio-potentials, require a high-impedance buffer amplifier. Although several semiconductor manufacturers offer amplifier ICs featuring low bias and offset-input currents, attaching a sensor cable to an amplifier circuit can inflict damage from ESD (electrostatic discharge). Figure 1 shows one unsatisfactory approach to ESD protection. Resistor R1 limits an ESD event's discharge current, and diodes D1A and D1B clamp amplifier IC1's input to its power-supply rails. Unfortunately, when shunting a pH sensor's 400-MΩ input impedance, even low-leakage diodes, such as Fairchild Semiconductor's MMBD-1503A, introduce significant offset voltages.

　　The circuit in Figure 2 offers an alternative approach. An Analog Devices low-input-bias, low-offset-current AD8603 amplifier, IC1, serves as a unity-gain input buffer. For any normal input, the circuit's output voltage, VOUT, equals its input voltage, VIN. Thus, the voltage across ESD-protection diode D1A or D1B approaches 0V, and neither diode's leakage current affects the sensor's output signal. Depending on the polarity of an ESD event you apply to the circuit's input connector, its high-voltage spike discharges through diode D1A or D1B into the positive or the negative power-supply rail. Capacitor C1 acts as an intermediate charge reservoir that slows the ESD spike's rate of rise and protects IC1's output stage from latching until diode D2A or D2B begins diversion of the ESD transient into the positive or the negative supply rail. In effect, C1 compensates for D1's parasitic capacitance. Resistor R3 allows IC1 to drive the capacitive load that C1 presents without going into oscillation.

　　During an ESD event, both D1 and D2 can conduct, but the voltage at VIN exceeds the power-supply-rail voltage by only two forward-biased diode voltage drops. Resistors R1 and R2 limit the amplifier input's currents below the manufacturer's recommended 5-mA maximum rating.

　　When packaging the circuit, pay special attention to the pc board's layout. Imperfections in the board's dielectric properties can provide parasitic-leakage-current paths. Adding copper traces on both sides of the board to form guard rings around the circuit's high-impedance nodes diverts leakage currents (Figure 3).