Datasheet

+
+ VIN
I
I
2
1
V =
LOGOUT
0.375 LOG
10
·
+ 2 V + I 50
1
W·
( (
I
I
2
1
V = [
LOG
0.375 LOG
10
·
+ I 50 ]
1
W· · 0.3 V/V + 2 V
( (
(I 3)
1 k
1
W
·
2
21.5 kW
+
5 kW·V = 2 +
ERR
V = 2 V V
OUT LOG ERR
-·
Connecting to the LOG114EVM www.ti.com
In the case where I
1
= 500 nA = I
REF
, the V
LOGOUT
terminal is equal to the level shifting bias voltage of 2 V
and the V
OUT
terminal is equal to 1.5 V. The circuit in Figure 10, while convenient for creating an input
current, does not take advantage of the high-current linearity correction circuitry. To use this circuitry, a
second power supply must be used and configured as shown in Figure 11.
Figure 11. Using a Resistor and the High Current Linearity Correction Circuitry
Changing the input voltage or the resistor value changes the input current I
1
(I
1
= V
IN
/R) and results in a
change for the V
LOGOUT
terminal voltage according to Equation 5.
(5)
For input currents less than 1 mA, this equation simplifies to Equation 2.
The voltage at the V
LOG
terminal is given by Equation 6:
(6)
The voltage at the V
ERR
terminal is given by Equation 7:
(7)
The voltage at the V
OUT
terminal is given by Equation 8:
(8)
10
LOG114EVM User Guide SBOU110– September 2011
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