Method for amending the effect of offset voltage variations from the output voltage in a D/A converter

The present invention relates to an offset voltage zeroing method of a D/A converter, with the aid of which oscillations of the output voltage of said D/A converter can be radically reduced. An output signal of the converter is carried to a first terminal of a capacitor (5). A second terminal of the capacitor is coupled to a reference voltage (V.sub.ref) with a controllable switch (6) as long as there are no pulses in the output of the converter. As a result, an amended output signal is obtained from the second terminal.

The present invention relates to an offset voltage zeroing method with 
which the output voltage variations of a D/A converter can be essentially 
reduced, said variations being caused by changes according to the 
temperature in offset voltages of the converter. The invention also 
relates to a circuit for implementing the procedure. 
Especially in D/A converters using the CMOS technology, the problems are 
related to variations of the output voltage at different temperatures, 
these being on one hand dependent on variations in the reference voltage 
and on the other, on the drifting of the offset voltage of the operational 
amplifiers used in the implementation of converters. The reference voltage 
could, in principle, be formed using circuits external to the converter, 
but that would lead to an increase in the number of components and in 
additional costs. In addition, changes in the offset voltage would still 
cause problems. 
In digital radio telephones, a transmission is carried out in the form of 
bursts so that a signal to be transmitted consists of subsequent pulses. A 
rise and decay of a transmission pulse should not be stepwise but the rise 
and decay are expected to be controllable so that the transmission 
spectrum would not spread to be too wide; nevertheless, the rise and decay 
times should be as short as possible. The cos.sup.2 waveform has often 
been considered to be an appropriate rising and decaying pulse shape. The 
timing of a transmission pulse should also be independent of the power 
level of the transmitter. 
In the Patent Application No. FI-896266 "(A power control method for a 
voltage controlled power amplifier and circuitry used in the same") a 
power control method of a transmitter based on the use of two control 
signals TXC and TXP is disclosed. The block diagram of the transmitter of 
a digital radio telephone according to said method is presented in FIG. 1. 
In the circuit the input square wave pulses P.sub.in are amplified as 
desired and cos.sup.2 -transformed, as to the rise and decay, into output 
powered pulses P.sub.out in a multistage power amplifier 1. The output 
power is measured with the aid of a direction coupler 2 and a power sensor 
3 from which a voltage V1 comparable to the power is obtained. Said 
voltage is carried to a control amplifier 4 being e.g. an operational 
amplifier, and a control pulse TXC enters a second input of the control 
amplifier 4 from the D/A converter. The control loop includes resistors 
R1, R2 and R3. With the output voltage of the control amplifier 4 a square 
wave pulse TXP is summed after the resistor R3, the height of said pulse 
being about as great as the voltage after which there is power at the 
output of the power amplifier 1. The square wave pulse starts 
substantially at the same moment as the control pulse TXC starts to rise. 
Now the capacitor C1 is charged through the resistor R4 to approach 
rapidly the threshold level, and the control loop of the power amplifier 1 
is operative as soon as there is power at the output of the power 
amplifier 1. 
If the actual control signal TXC affecting the output power has been 
produced e.g. with an ordinary inexpensive CMOS-D/A converter, the output 
power changes together with the changing temperature. This is due to 
changes of the reference voltage and offset voltage in the D/A converter 
described above. 
Very few means for eliminating said problem are available in the art. An 
individual temperature compensation can be carried out in every radio 
telephone, which causes extra work. Alternatively, high-standard and more 
expensive D/A converters may also be used. 
The object of the present invention is to provide an offset voltage zeroing 
procedure of a D/A-converter with which the above described problems can 
be solved. For achieving said aim, the present invention is characterized 
in that the variations of the offset voltage are eliminated from a TXC-D/A 
signal by adding a serial capacitor and by keeping the end of the 
capacitor facing the TXC circuit with the aid of a switch at a reference 
voltage V.sub.ref as long as there is no pulse in the TXC-D/A signal.

FIG. 1 is described above. The design of the invention is described below 
referring to FIGS. 2 to 4 which show an embodiment of the design according 
to the invention. 
FIG. 2 shows the operation principle of an offset voltage zeroing system of 
a D/A converter according to the invention. A TXC-D/A signal is a pulse 
derived from a D/A converter, being in shape cos.sup.2 as regards its rise 
and decay, intended for controlling the transmitter 1 shown in FIG. 1, and 
the TXC signal refers to a pulse obtained from a TXC-D/A signal to be 
carried to the power control loop of a transmitter, and being cos.sup.2 
-shaped regarding the rise and decay, wherefrom the variations of the 
offset voltage have been removed. The TXP signal is a square wave pulse 
used in accelerating the power control circuit in the manner described in 
the Finnish Patent application FI-896266. The changes in the output 
voltage TXC-D/A of a D/A converter, caused by variations in the offset 
voltage, can be removed from the TXC-D/A signal by adding a serial 
capacitor 5 and by maintaining the end of the capacitor 5 facing the TXC 
terminal with the aid of a switch 6 at the reference voltage V.sub.ref as 
long as there are no pulses in the TXC-D/A signal. As soon as a pulse 
TXC-DA enters, the switch opens and V.sub.ref is switched off from the TXC 
terminal of the capacitor 5. The pulse may now travel unchanged through 
the capacitor 5, and after the pulse ends, the voltage of the TXC terminal 
is switched back to the level of the reference voltage. The switch 6 is 
controlled via an inverter 7 with a TXP pulse. 
FIGS. 2-4 are related to an embodiment of the invention in controlling the 
transmitter of a radio telephone, but the invention may also be employed 
otherwise in stabilizing the output voltage of a D/A converter. 
If the switch 6 were not at all used, the voltage level would glide after 
the capacitor 5 up and down, being dependent, as regards the rise and 
decay, on the level of the cos.sup.2 shaped pulse. If the switch 6 is 
zeroactive in operation, no inverter 7 is needed. No inverter 7 is needed, 
either, if an inverted TXP pulse is used in the power control loop. 
The same offset voltage zeroing principle may also be used in applications 
using the time-sharing principle, since the offset voltage can be zeroed 
without being included in the actual operational cycle. 
FIG. 3 shows the curve shapes of the signals employed in the procedure of 
the invention. The dashed line presents the variation of the TXC-D/A 
output voltage 8 of a D/A converter caused by the offset voltage. The TXP 
voltage is indicated by numeral 9 and the TXC voltage by numeral 10. 
FIG. 4 shows the circuit diagram of the method according to the invention. 
For the inverter 7, a transistor 11 is used and for the switch 6, a 
transistor 12. The TXP square wave pulse has been carried to the base of 
the transistor 11 through a resistor 13. The base of the transistor 12 has 
been coupled to the positive operating voltage through a resistor 14. The 
reference voltage has been produced from the operating voltage with the 
aid of voltage divider resistors 15 and 16. The reference voltage may also 
be zero, resulting in omission of the resistors 15 and 16, and in 
grounding the emitter of the transistor 12. 
With the aid of the invention, D/A converters of prior art technology can 
be used. Only a few additional components are needed in the design, and it 
removes the need of tuning the stages controlled with a D/A converter. 
No individual temperature compensations of radio telephones are needed and 
also advantages related to production are achieved, such as improvement in 
the gain. Most part of the offset voltage zeroing coupling can be 
integrated within a D/A converter circuit. 
Compensation of the offset voltage of a D/A converter is described above 
with an example concerning the control pulse of a radio phone transmitter 
implemented by means of the digital time sharing principle. It is obvious 
that this principle can be used in any embodiment in which one wishes to 
eliminate the effect of the offset voltage from the out put voltage of a 
D/A converter. The only prerequisite for said application is to find out 
when a pulse from the converter starts and ends and that this information 
can be used for controlling the switch. Said information can be obtained 
from another pulse which preferably starts just a bit before the start of 
a pulse from the converter and ends simultaneously therewith. The 
invention is particularly appropriate for use in a radio telephone 
operating on the time division principle.