N 3 N_3 N3高, N 4 N_4 N4低: S a 1 S a 2 S_{a1} S_{a2} Sa1Sa2接通, S b 1 S b 2 S_{b1} S_{b2} Sb1Sb2断开, 电路失调调零, N 1 N_1 N1输入端仅有失调电压 U o s 1 U_{os1} Uos1
U C 1 = − ( U o 1 + U o s 2 ) K 2 U_{C1}=-(U_{o1}+U_{os2})K_2 UC1=−(Uo1+Uos2)K2
U o 1 = − ( − U o s 1 + U C 1 ) K 1 U_{o1}=-(-U_{os1}+U_{C1})K_1 Uo1=−(−Uos1+UC1)K1
K 1 K_1 K1: N 1 N_1 N1闭环放大倍数 K 2 K_2 K2: N 2 N_2 N2开环放大倍数 >>1
U C 1 ≈ U o s 1 − U o s 2 K 1 ≈ U o s 1 U_{C1}\approx U_{os1}-\frac{U_{os2}}{K_1}\approx U_{os1} UC1≈Uos1−K1Uos2≈Uos1
N 3 N_3 N3低, N 4 N_4 N4高: S a 1 S a 2 S_{a1} S_{a2} Sa1Sa2断开, S b 1 S b 2 S_{b1} S_{b2} Sb1Sb2接通, 信号放大状态
U o = − U i R 2 R 1 − U o s 1 K 1 + U C 1 K 1 ≈ − R 2 R 1 U i U_o=-U_i\frac{R_2}{R_1}-U_{os1}K_1+U_{C1}K_1\approx -\frac{R_2}{R_1}U_i Uo=−UiR1R2−Uos1K1+UC1K1≈−R1R2Ui
低漂移集成运放
轮换自动校零集成运放
模拟开关切换, 使内部两个性能一致的运放 N 1 N 2 N_1 N_2 N1N2交替工作在信号放大与自动校零两种状态
N 1 N_1 N1信号放大, 则 N 2 N_2 N2自动校零, 如左图 N 2 N_2 N2无信号输入, C 2 C_2 C2寄存了 N 2 N_2 N2的输入失调和低频瞬时干扰电压(矫正电压); 当1转换为信号放大状态, N 1 N_1 N1为自动校零状态, C 2 C_2 C2的矫正电压抵消了 N 2 N_2 N2的输入失调和低频瞬时干扰电压, 自动校零, N 2 N_2 N2输出放大了的输入信号. N1同理
U − = U o s , U + = − K U a U_-=U_{os}, U_+=-KU_a U−=Uos,U+=−KUa
U a = Δ U o R 1 / ( R 1 + R 5 ) U_a=\Delta U_o R_1/(R_1+R_5) Ua=ΔUoR1/(R1+R5)
Δ U o = − K 1 ( U − − U + ) ≈ − ( U o s / K ) ( 1 + R 5 / R 1 ) \Delta U_o=-K_1(U_--U_+)\approx -(U_{os}/K)(1+R_5/R_1) ΔUo=−K1(U−−U+)≈−(Uos/K)(1+R5/R1)
时钟高: 误差检测&寄存
时钟低: 校零&放大, 消除了失调电压与共模电压
C M R R = K 2 C M R R CMRR=K_2 CMRR CMRR=K2CMRR 共模抑制比提高 K 2 K_2 K2倍
