The OPA862 is laser trimmed for high DC precision. An important factor that reduces the DC precision of the system that uses the OPA862 is the errors introduced by the bias currents of A2 flowing through the internal feedback resistors, R_INT; see Section 7.2. To minimize the error contribution from I_B, the A2 amplifier of the OPA862 features a unique I_B cancellation mechanism. This I_B cancellation mechanism is the reason why the I_B of A2 is orders of magnitude lower than the I_B of A1. The DC errors are negligible for most applications because of the nanoamperes of I_B and very low I_B drift of A2. However, despite being very low, if the I_B errors of A2 are significant for an application, a 348-Ω R_REF resistor can be used on the VREF input to cancel out the I_B errors. The tradeoff of using the R_REF is that this resistor introduces noise that is amplified by a factor of two at VOUT– because of the noise gain of two of A2. The C_FILT capacitor (see Section 7.2) also helps filter out the flat band noise contribution of R_REF. The 700-Ω internal resistors were carefully chosen to balance low noise while keeping the total power dissipation low by taking advantage of the low 3.1-mA quiescent current of the OPA862. As shown in Figure 7-3, to get the equivalent 8.3-nV/√ Hz noise of the OPA862 with a typical FDA configuration, the FDA must be less than 1 nV/√ Hz; such FDAs are often difficult to find or expensive. When R_REF equals 0 Ω, the typical error resulting from the I_B of A2 appears as an input-referred offset of 3.5 µV at the VREF input, and when R_REF is 348 Ω, the differential output-referred noise increases from 8.3 nV/√ Hz to 9.6 nV/√ Hz.

Figure 7-3 Equivalent Voltage Noise FDA to OPA862