Wide dynamic range linear to log converter with microcomputer control

The effective range of an analog linear-log converter is extended above the number of output decades over which this converter is reasonably linear to cover several additional decades of log conversion. This is made possible by adding a variable gain amplifier having several decades of switchable gain to the input of the converter. Switching between decades to utilize the added decades of output is controlled by a microcomputer which monitors the output level. This switching restores the signal level of each of the added decades to the same level as that of an earlier decade thus maintaining the same slope accuracy and noise levels of the earlier decades throughout the whole conversion range. The microcomputer also provides slope corrections from a lookup table for each of the different output decades as well as also applying to the output corrections for offsets due to the circuit components. Ambient temperature compensation is also provided following the linear-log converter for increased precision.

BACKGROUND OF THE INVENTION 
The linear-log converter in its analog form has been extensively used in 
spectrophotometry for absorbance measurements. In this form advantage is 
taken of the logarithmic characteristics of the base-emitter junction of a 
transistor used in the feedback link of an operational amplifier to 
convert a linear amplifier input into a log output. However, a range of 
over two decades of input begins to see deviation from the log 
characteristic. 
With modern spectrophotometers using holographic gratings and much reduced 
stray light a wider log, i.e. absorbance, output is essential. Digital 
performance of the linear-log conversion is limited by digitation noise in 
the analog to digital converter. 
BRIEF DESCRIPTION OF THE INVENTION 
According to the invention there is provided a ratiometric linear to log 
converter with a preselectable dynamic range of up to 7 absorbance units 
having excellent conformity. This system utilizes state of the art solid 
state technology and the power of the microprocessor for all the required 
adjustments and data manipulations. It also eliminates the need for a high 
resolution A/D converter for higher performance instruments. At this time 
the 16 bit resolution A/D converter has the highest resolution that has 
been used on these sophisticated instruments. At 16 bits resolution for an 
A/D converter a digital noise of 0.0065 absorbance is expected at 3.0 
absorbance. Increasing the resolution beyond 16 bits is very costly. 
Utilizing the described system of this invention, a resolution of only 12 
to 14 bits will be required to perform measurements of up to 7 decades 
absorbance. 
It is an object of this invention to combine state of the art analog 
electronics with the power of a microcomputer to provide a wider linear to 
log conversion range than is possible with either analog or digital 
conversion alone. 
It is a further object of this invention to provide, through microcomputer 
control, automatic calibration and linearity correction. 
It is yet a further objective to minimize quantization noise at high 
absorbance values. 
It is a still further objective to eliminate the need for a high 
resolution, expensive A/D converter. 
It is also an object to accomplish the above while still providing a fast 
conversion. 
The system comprises a programmable gain instrumentation amplifier followed 
by a linear to log converter. The output of the linear to log converter, 
after further amplification, is delivered through an A/D converter to a 
microcomputer for data processing. Over the highest two decades of input 
signal the gain of the programmable gain amplifier is set at unity. On 
entering the next lower input decade the microcomputer responds by 
increasing the amplifier gain to 10 and revising the output signal to the 
next higher decade of absorbance. The same procedure takes place on 
entering each of the next lower decades of input, the gain of the 
amplifier becoming 100 and 1000, respectively. Corrections are made in 
each decade for any change of slope of the conversion function. By this 
procedure a range of five decades can be covered; the lower three being 
under the control of the microcomputer. If desired this procedure can be 
further extended to six or seven decades. Furthermore, the microcomputer 
also may be used to initially calibrate the system, determining 
automatically and then storing corrections for offsets and slope variation 
from decade to decade. In this way a high degree of correction for the 
linear to log conversion is attained. 
There has thus been outlined rather broadly the more important features of 
the invention in order that the detailed description thereof that follows 
may be better understood, and in order that the present contribution to 
the art may better appreciated. There are, of course, additional features 
of the invention that will be described more fully hereinafter. Those 
skilled in the art will appreciate that the conception on which this 
disclosure is based may readily be utilized as the basis of the designing 
of other apparatus for carrying out the various purposes of the invention. 
It is important, therefore, that this disclosure be regarded as including 
such equivalent apparatus as do not depart from the spirit and scope of 
the invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENT 
Referring to the FIGURE, input Vi to the system is applied through a 
variable gain instrumentation amplifier 12, which may accept either a 
single-ended input or two variable inputs. In the preferred embodiment 
Analog Devices AD524 is used although equivalent amplifiers may 
alternatively be used. In this type of amplifier, gain is programmable by 
connecting together selected pins as indicated at 13. These connections 
may be made, as will be evident to one skilled in the art, by digital 
switching devices, relays, or gates connected to pins 13 controlled from 
the microcomputer unit (MPU) 14 through suitable peripheral interface 
adaptors 34. The gain of amplifier 12 can be set at unity, x10, x100 or 
x1000, for example. All the resistors for gain control are part of the 
same substrate as the amplifier proper for maximum accuracy and 
temperature tracking. 
A linear-log conversion amplifier 10 is provided which includes a dual 
operational amplifier 16. The data output Vs of variable gain amplifier 12 
is directed to the input of one half 16A of the dual operational amplifier 
16. Since both halves of this dual amplifier are in the same housing all 
temperature sensitive characteristics will be alike for both. The 
amplifier used in the illustrated embodiment is a AD644, Analog Devices, 
although other dual amplifiers, presenting a similar very low zero offset 
voltage, low temperature drift and low input bias current, may be used. 
The second half of this dual amplifier 16B receives its input from a 
reference voltage V.sub.R. In the feedback of each half of amplifier 16 is 
connected one-half 18A and 18B of a dual transistor. For example, the 
transistors used in this embodiment are Motorola MP318, silicon planar 
transistors having very high DC gain, low ohmic resistance and matched 
parameters. When connected as shown the base to emitter junction exhibits 
a characteristic logarithmic relationship between voltage and current. The 
two transistors being in the same mount are maintained both at the same 
temperature. The output V.sub.os of each half of this linear-log 
conversion amplifier 10 will have a logarithmic relationship to linear 
input voltage changes. 
To eliminate initial offset voltages a difference amplifier 22, also AD644, 
follows and accepts the outputs of amplifier 10. This amplifier is 
connected for unity gain whereas the following output amplifier 24 has a 
fixed gain. The output of the data half of the log amplifier 16A is 
connected to the positive, non-inverting terminal of amplifier 22 and the 
output of amplifier 22 is similarly connected to the input of amplifier 
24. The output of the reference log amplifier 16B is connected to the 
inverting input of amplifier 22. Since both amplifiers 16A and 16B are on 
the same substrate, the zero offset and other temperature characteristics 
of both will be equalized; hence, only the log signal output will be left 
after the output of amplifier 16B is subtracted from the output of 
amplifier 16A by the difference amplifier 22. 
The theory of linear to logarithmic conversion by using a transistor as a 
trans-diode in the closed feedback loop of an operational amplifier is 
well known in the art and need not be discussed in detail here. It can be 
shown that the output voltage V.sub.OD of the difference amplifier 22 
equals E.sub.T log (I.sub.S /I.sub.R), where I.sub.S is the input current 
to amplifier 16A, and I.sub.R is the input current to amplifier 16B. 
E.sub.T =KT/q, where K is Boltzmann's constant, T is absolute temperature 
in Kelvins, q is the charge on an electron. A change in ambient 
temperature will cause an output change of 0.3 percent per degree C. To 
compensate for this change, the gain of the last stage 24 is varied 
inversely to the temperature by using a silicon resistor 26 of positive 
temperature coefficient of 0.7 percent per degree C. in series with a 
suitable resistor 28 in the inverting input leg of amplifier 24. 
The output V.sub.o of amplifier 24, corrected for temperature, is applied 
to an A/D converter 30. In the illustrated embodiment any suitable 12 to 
14 bit converter of adequate speed for the specific application may be 
used. The digitized signal from the A/D converter is forwarded to the 
microprocessor unit 14 (MPU) for data processing. 
The operation of the described system in conjunction with the MPU to attain 
the objectives of this invention may be described in detail as follows: 
When an input voltage V.sub.S (say, 10.0 V) representing 100 percent 
transmittance is applied to the input of amplifier 12 and a fixed voltage 
V.sub.R (10.0 V) is applied to the input of the linear-log converter, 10, 
the output voltage V.sub.O will be 0.00 V representing a zero absorbance 
measurement. 
As the input voltage or transmittance at the input decreases by one decade, 
(V.sub.S =1.00 V and V.sub.R =10.0 V), the output voltage V.sub.O will 
indicate 1.00 V or 1.00 absorbance. 
As the input voltage decreases by another decade (V.sub.S =0.10 V) the 
absorbance changes to 2.00 A. 
For a system of dynamic range of five absorbance level, the maximum dynamic 
range of the linear-log converter need be only two absorbance in 
conjunction with the gain switching amplifier. To obtain a dynamic range 
of five absorbance units from a two decade linear-log converter the 
following process will occur: 
As the input voltage or transmittance decreases and the output voltage 
V.sub.O or absorbance increases, say, to 2.001 A, the microcomputer 
detects the 2.001 A in digital form and automatically changes the gain of 
the amplifier 12 by one decade, i.e. to x10, thereby increasing the input 
voltage (transmittance) to the linear-log converter by a factor of ten and 
decreasing the output voltage (the indicated absorbance) by one decade 
back to 1.001 A. To correct the indicated absorbance value the MPU 
utilizes a look-up table and selects a proper absorbance readout of 2.001 
A. This will then be the indicated value. To minimize the fluctuations 
caused by noise during the transition from one decade to the next, a delay 
or hysteresis loop may be included in the circuit. 
Once again, as the transmittance decreases by another decade and the 
absorbance reaches a new value of 3.001 A, the MPU detects the 3.001 A and 
automatically changes the gain of the amplifier 12 by another decade, to 
x100, thereby increasing the input voltage to the linear-log converter by 
x100 and decreasing the output absorbance by two decades, from 3.001 to 
1.001 A. At this time the MPU utilizes the look-up table again and 
indicates an absorbance of 3.001 A to the readout. 
The same process takes place at 4.001 A for 4.001 A to 5.00 A. It will be 
apparent that the 2.001 absorbance as a switching point could be set at 
3.001 or 4.001 to increase the dynamic range to 6 or 7 decades absorbance 
with deviation from the exact linear-log conversion characteristic being 
corrected digitally by the MPU from look-up tables. Therefore, the 
switching points or the dynamic range are preselectable under computer 
control. 
For the above described operating sequence to function properly and 
accurately a calibration routine controlled by the MPU must initially be 
performed. This may be done when the instrument is started up or at any 
desired later time. 
The calibration process takes place in a sequential form as follows: 
1. Switch SW-1 is operated by the MPU command to apply a voltage of 10.0 V 
to the input of amplifier 12 via DAC-1, 32. The gain of amplifier 12 is 
set to unity by the MPU. The input to the reference side 16B of the 
linear-log converter 10 is a fixed voltage, also 10.0 V. The output of 
amplifier 24 is converted into digital form by the A/D converter 30 in 
conjunction with the MPU 4. Since the inputs to data and reference sides 
of amplifier 10 are both 10.0 V, the output should indicate 0.00 V zero 
absorbance. If the output indicates any value other than zero, the 
difference is registered in the MPU. This difference is added 
algebraically to the subsequent readings as an offset voltage correction. 
This offset voltage correction compensates for the differences between 
sample and reference voltages and also between the offset voltages of all 
the related amplifiers. 
2. A voltage of 1.00 V is then applied via DAC-1, 32, to the input of 
amplifier 12 at unity gain; the output should as a result indicate 1.00 V 
or 1 A. If the output indicates other than 1.00 V (1.00 A), the difference 
is noted by the MPU and a multiplying factor is determined to apply to the 
data as a slope factor to make the output read exactly 1.00 V ie. 1.00 A. 
3. The input voltage to amplifier 12 is then reduced by another decade via 
DAC-1, 32, the gain of amplifier 12 is switched from x1 to x10 by the MPU 
(assuming a five decades logarithmic operation), and the output of the A/D 
converter 30 is noted by the MPU. If the output is different from 1.00 V, 
a new multiplying factor is calculated as a slope factor for the gain 
setting of x10 of the amplifier 12. 
4. The input voltage to amplifier 12 is further reduced two more decades 
via DAC-1, 32, in a similar sequential form and each time a new slope 
factor is calculated for each decade. 
Therefore, any gain error caused by the amplifier 12 and the converter 10 
is monitored and compensated for by the MPU. This calibration process 
eliminates the need for any manual calibration procedure since corrections 
for all variables and offsets are handled by the MPU as directed by 
appropriate software routines. 
Although a certain particular embodiment of the invention has been herein 
disclosed for purposes of explanation, various modifications thereof, 
after study of the specification, will be apparent to those skilled in the 
art to which the invention pertains, and reference should accordingly be 
had to the appended claims in determining the scope of the invention.