Application Hints (Continued)
In very critical applications where a manual adjustment is
impracticalꢂ the LMC669 Auto Zero circuit may be used to
reduce the effective input offset voltage to around 5 mV as
in Figure 4bꢁ The LF400 will perform better than slower am-
plifiers in an auto zero loopꢂ because its fast settling capabil-
ity keeps its summing node voltage more stableꢁ Thereforeꢂ
the LMC669 is able to more accurately sample the summing
node voltage before making an offset correctionꢁ
Input Bias Current
The JFET input stage of the LF400 ensures low input bias
current (200 pA maximum) when the die is at room tempera-
tureꢂ but this current approximately doubles for every 10 C
TLꢀHꢀ9414–24
FIGURE 4aꢃ V Adjust Circuit
os
ꢀ
increase in temperatureꢁ In applications that demand the
lowest possible input bias currentꢂ a heat sink should be
used with the LF400ꢁ ‘‘Press on’’ heat sinks from manufac-
turers such as Thermalloy and AAVID can reduce junction
temperature by roughly 10 C to 40 Cꢁ
ꢀ
ꢀ
Typical Applications
High-Speed DAC with Voltage Output
TLꢀHꢀ9414–26
TLꢀHꢀ9414–25
FIGURE 4bꢃ Automatic Offset
Adjustment Using LMC669
11
NSC [ National Semiconductor ]
