Datasheet MAX4100, MAX4101 (Maxim) - 9

500MHz, Low-Power Op Amps MAX4100/MAX4101
RG RF
Table 1. Resistor and Bandwidth Values
VIN
for Various Gain Configurations BANDWIDTH GAIN RG RF LIMIT* (V/V) (
Ω
) (
Ω
)
C MAX4100 MAX4100 V
(MHz)
OUT MAX4101 MAX4101 +1 ∞ 24 1659 +2 200 200 398 +5 50 200 995 +10 30 270 1474 Figure 2. Effect of Feedback Resistor Values and Parasitic Capacitance on Bandwidth -1 200 200 398 -2 75 150 796
Setting Gain
-5 50 250 955 The MAX4100/MAX4101 are voltage-feedback op amps that can be configured as an inverting or noninverting -10 50 500 875 gain block, as shown in Figures 1a and 1b. The gain is * Assuming an infinite bandwidth amplifier. determined by the ratio of two resistors and does not affect amplifier frequency compensation.
Resistor Types
Surface-mount resistors are the best choice for high- In the unity-gain configuration (as shown in Figure 1c), frequency circuits. They are of similar material to the maximum bandwidth and stability is achieved with the metal film resistors, but are deposited using a thick-film MAX4100 when a small feedback resistor is included. process in a flat, linear manner so that inductance is This resistor suppresses the negative effects of para- minimized. Their small size and lack of leads also mini- sitic inductance and capacitance. A value of 24Ω pro- mize parasitic inductance and capacitance, thereby vides the best combination of wide bandwidth, low yielding more predictable performance. peaking, and fast settling time. In addition, this resistor reduces the errors from input bias currents.
DC and Noise Errors Choosing Resistor Values
There are several major error sources to be considered in any operational amplifier. These apply equally to the The values of feedback and input resistors used in the MAX4100/MAX4101. Offset-error terms are given by the inverting or noninverting gain configurations are not equation below. Voltage and current noise errors are critical (as is the case with current feedback ampli- root-square summed, so are computed separately. fiers). However, take care when selecting because the Using the circuit in Figure 3, the total output offset volt- ohmic values need to be kept small and noninductive age is determined by: for practical reasons. a) The input offset voltage (V The input capacitance of the MAX4100/MAX4101 is OS) times the closed-loop gain (1 + R approximately 2pF. In either the inverting or noninvert- F / RG) ing configuration, the bandwidth limit caused by the b) The positive input bias current (IB+) times the package capacitance and resistor time constant is source resistor (RS) minus the negative input bias f3dB = 1 / (2Π RC), where R is the parallel combination current (IB-) times the parallel combination of RG of the input and feedback resistors (RF and RG in and RF. IOS (offset current) is the difference Figure 2) and C is the package and board capacitance between the two bias currents. If RG | | RF = RS, this at the inverting input. Table 1 shows the bandwidth limit part of the expression becomes IOS x RS. for several values of RF and RG, assuming 4pF total The equation for total DC error is: capacitance (2pF for the MAX4100/MAX4101 and 2pF of PC board parasitics).  R  V = I( R + V F ) OUT OS S OS 1+  RG 
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