7 200pF 3
15 12 R7 13
FIGURE 1. ICL8049 OFFSET AND SCALE FACTOR ADJUSTMENT
ICL8049 Detailed Description
The ICL8049 relies on the same logarithmic properties of
the transistor as the ICL8048. The input voltage forces a
specific ∆VBE between Q1 and Q2 (Figure 1). This VBE
difference is converted into a difference of collector currents
by the transistor pair. The equation governing the behavior
of the transistor pair is derived from (2) on the previous page
and is as follows:
II--CC----12-- = exp q----∆--k--V-T--B----E--
When numerical values for q/kT are put into this equation, it
is found that a ∆VBE of 59mV (at +25oC) is required to
change the collector current ratio by a factor of ten. But for
ease of application, it is desirable that a 1V change at the
input generate a tenfold change at the output. The required
input attenuation is achieved by the network comprising R1
and R2. In order that scale factors other than one decade
per volt may be selected, R2 is external to the chip. It should
have a value of 1kΩ, adjustable ±20%, for one decade per
volt. R1 is a thin film resistor deposited on the monolithic
chip; its temperature characteristics are chosen to
compensate the temperature dependence of Equation 1, as
explained on the previous page.
The overall transfer function is as follows:
Substituting VOUT = IOUT x ROUT gives:
VOUT = ROUT IREF exp (---R----1-–---R+----2-R----2----) × q----kV--T-I--N---
For voltage references Equation 3 becomes
VREF × -RR---OR---UE---FT- exp
ICL8049 Offset and Scale Factor
As with the log amplifier, the antilog amplifier requires three
adjustments. The first step is to null out the offset voltage of
A2. This is accomplished by reverse biasing the base-emitter
of Q2. A2 then operates as a unity gain buffer with a
grounded input. The second step forces VIN = 0; the output is
adjusted for VOUT = 10V. This step essentially “anchors” one
point on the transfer function. The third step applies a
specific input and adjusts the output to the correct voltage.
This sets the scale factor. Referring to Figure 1 the exact
procedure for 1 decade/volt is as follows:
1. Connect the input (pin #16) to +15V. This reverse biases
the base-emitter of Q2. Adjust R7 for VOUT = 0V. Discon-
nect the input from +15V.
2. Connect the input to Ground. Adjust R4 for VOUT = 10V.
Disconnect the input from Ground.
3. Connect the input to a precise 2V supply and adjust R2 for
VOUT = 100mV.
The procedure outlined above optimizes the performance
over a 3 decade range at the output (i.e., VOUT from 10mV)
to 10V). For a more limited range of output voltages, for
example 1V to 10V, it would be better to use a precise 1V
supply and adjust for VOUT = 1V. For other scale factors and/
or starting points, different values for R2 and RREF will be
needed, but the same basic procedure applies.