500MHz, Low-Power Op Amps In both DC and noise calculations, errors are dominat- ed by offset voltage and noise voltage (rather than by input bias current or noise current). Metal-film resistors with leads are manufactured using RI RF a thin-film process, where resistive material is deposit- ed in a spiral layer around a ceramic rod. Although the materials used are noninductive, the spiral winding pre- IB- sents a small inductance (about 5nH) that may have an adverse effect on high-frequency circuits. MAX4100 VOUT R I MAX4101 S B+ 10 8 RS = 0Ω MAX4100-4a CL = 10pF 6 4 MAX4100/MAX4101 Figure 3. Output Offset Voltage 2 0 c) Total output-referred noise voltage is shown by the -2 equation below (en(OUT)): CL = 5pF -4 R 2 2 CLOSED-LOOP GAIN (dB) C e F 1 i 2 R e -6 L = 0pF n OUT ( ) = + n S N RG ( ) +( ) -8 The MAX4100/MAX4101, with two high-impedance inputs, -10 0.1M 1M 10M 100M 1G have low 8nV√Hz voltage noise and only 0.8pA√Hz current FREQUENCY (Hz) noise. An example of DC error calculations, using the MAX4100/MAX4101 typical data and the typical operating Figure 4a. MAX4100 Bandwidth vs. Capacitive Load circuit with RF = RG = 200Ω (RS = 100Ω), gives: R V = I( R + V F ) OUT OS S OS 1+ RG 10 −6 2 −3 V 3 10 10 1 10 (1 ) C OUT = × × + × +1 8 L = 10pF MAX4100-4b 6 V = . 2 m 6 V OUT 4 2 Calculating total output-referred noise in a similar man- ner yields: 0 -2 − 2 2 -4 12 −9 R e (1 ) CLOSED-LOOP GAIN (dB) S = 22Ω n OUT ( ) = +1 2 × . 0 8 × 10 × 10 0 + 8 × 10 -6 RS = 10Ω RS = 4.7Ω -8 e = n 8 V / Hz RS = 2.2Ω n OUT ( ) -10 0.1M 1M 10M 100M 1G With a 200MHz system bandwidth, this calculates to FREQUENCY (Hz) 133µVRMS (approximately 679µVp-p). Figure 4b. MAX4100 Bandwidth vs. Capacitive Load and Isolation Resistor 10______________________________________________________________________________________
