To process this information with digital techniques it must first be converted from its analog to digital form. The device that does this is knows as an analog-to-digital converted (ADC or A/D converter).
In addition, since many types of electronic equipment are inherently analog devices (e.g., stereo amplifiers, radio and television receivers), there are many occasions when it is necessary to transform digital information to analog information. This is accomplished by using a device know as a digital-to-analog converter (DAC or D/A converter).
The Operational Amplifier
The operational amplifier, referred to as the op-amp for short, is a linear amplifier that has two inputs (inverting and noninverting) and one output. The op-amp has a very high voltage gain and a very high input impedance, as well as a very low output impedance. The op-amp symbol is shown in Figure 5-1, note that the noninverting input is indicated by '-' and the inverting input by '+'.
Figure 5-1 Operational Amplifier
When used as an inverting amplifier, as indicated in Figure 5-2, the feedback resister and the input resister control the voltage gain as indicated in Equation 5-1.
Figure 5-2 Op-amp as an inverting amplifier
Equation 5-1
This is known as the closed-loop voltage gain because it refers to the feedback from output to input provided by RF.
In the inverting amplifier configuration, the inverting input is approximately at ground potential (0 V). This is due to the inherent op-amp properties, the feedback mechanism and the connection of the noninverting input to the ground.
When the op-amp is used as a comparator, as shown in Figure 5-3, two voltages (VIN1 and VIN2) are applied to its inputs. When these voltages differ by a very small amount, the op-amp is driven into one of its saturated states, depending on which input voltage is greater.
Figure 5-3 Op-amp as a comparator
SAMPLING
Sampling is the technique of controlling the time at which information will be converted. Both digital and analog signals may be sampled. In particular, lets examine the case where analog signals are sampled. Figure 5-4 shows the voltage follower sample-and-hold circuit.
Figure 5-4 Sample-and-hold circuit
The input signal charges the capacitor (C) and through the resistor (R) when the switch is in the "sampling" position. At the desired instant, the switch is changed to the "hold" position isolating the input signal and leaving the input potential, at that instant, across the capacitor at the amplifier input. Ideally, this voltage would be maintained (held) indefinitely at the noninverting (+) input and consequently at the output. However, finite currents at the amplifier inputs or through the sampling switch cause the voltage across C to change with time. An ideal sample-and-hold circuit would have infinitesimal sample and hold times.
Digital-to-Analog Converters
A simple 3-bit D/A converter is illustrated in Figure 5-5. The input to the circuit is the 3-bit binary number A2A1A0, where each of the variables takes the value of 0V or 1V and A2 represents the MSB.
Figure 5-5 Three-bit DAC circuit
Let us take a look at the operation of this circuit:
The resolution is the reciprocal of the number of discrete steps in the D/A output.
The accuracy is a comparison of the actual output of a D/A converter with the expected output.
- Linearity is a deviation from the ideal straight-line output of a D/A converter
There are a number of approaches that can be taken for the construction of an analog-to-digital converter. One technique incorporates a DAC as shown in Figure 5-6. For this device, an analog input voltage is applied to one if the inputs of a voltage comparator.
Figure 5-6 Counting ADC
Circuit Operation
As long as the DAC output is applied to the other input of the comparator, and as long as the DAC output voltage is less than the analog input voltage, the comparator output is HIGH (logical 1) and the clock pulse is applied to the binary counter.
When the count of the binary counter is high enough such that the DAC output value exceeds that of the analog input, the comparator output is LOW (logical 0). Now, additional clock pulses are not applied to the counter and it stops counting.
The output of the counter, 
, forms the resulting n-bit digital output.
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The counter is then reset to zero (provision for this is not shown in the figure). |
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It is assumed that the analog input remained constant during the interval required for the counter to reach its final state. In practice a sample-and-hold circuit is required to ensure this is indeed the case. |
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This type of converter is also know as the Stairstep-Ramp A/C converter |
Question: What disadvantages are there with the counting ADC?
Note: Binary counter used in the ADC converter is a binary up counter. If this counter is replaced with a binary up/down counter then the converter is referred to as a tracking ADC converter. When a binary '1' is input to the counter it counts up. The reverse occurs when binary '0' is input.
Question: Can you determine the operation of the tracking ADC converter?
Flash converter ADC
The flash (simultaneous) A/D converter uses several voltage comparators that compare reference voltages with the analog input voltage. Figure 5-7 shows a two-bit parallel-comparator ADC converter. Note that no comparator is needed for the all-0s condition. In general a 2n-1comparators are needed for conversion to a n-bit binary code. The advantage of this circuit over the one illustrated in Figure 5-6 is that it provides a faster method of analog-to-digital conversion.
Figure 5-7 Flash ADC
Circuit operation
When the analog voltage exceeds the reference voltage for a given comparator, a HIGH is generated. The output of each comparator is connected to the input of the encoder which is sampled by a pulse on the enable input. The output from the encoder is the digital value of the analog input.
Digital Temperature Sensors
TI's high-accuracy, low-power
temperature sensors are specified
for operation from –40°C to +125°C
and are designed for cost-effective
thermal measurement in a variety of
communication, computer, consumer,
industrial and instrumentation
applications.
These silicon-based temperature sensors
are designed on a unique topology that
offers excellent accuracy and linearity
over temperature. Low power and
standard communication protocol pair
nicely with low-power microcontrollers
and battery-powered designs.
The digital temperature output of the
TMP family is created using a highperformance,
12-bit, delta-sigma
ADC that outputs temperatures as
a digital word. Programming and
communication with the TMPxxx family
of devices is done via an I2C/2-wire
interface or SPI interface for easy
integration into existing digital systems.
Temperature Sensor Core
A typical block diagram of the TMP
family of digital temperature sensors is
shown below. Temperature is sensed
through the die flag of the lead frame.
The temperature sensing element
is the chip itself, ensuring the most
accurate temperature information
of the monitored area and allowing
designers to respond quickly to "over"
and "under" thermal conditions.
Features of TMP Digital
Temperature Sensors
Several TMP digital sensors offer
programmable features, including overand
under-temperature thresholds,
alarm functions and measurement
resolution. With extremely low power
consumption in active (50μA) and
standby (0.1μA) modes, the TMP12x
family offers as low as 1.5°C minimum
error in a SOT23 package and is an
excellent candidate for low-power
thermal monitoring applications.
The new TMP105 and TMP106 are the
world's smallest digital temperature
sensors. Available in a tiny 1mm x
1.5mm chipscale package, they use
only 50μA of current and are ideal for
portable applications including mobile
phones, portable media players, digital
still cameras, hard disk drives, laptops,
and computer accessories. TMP105
has 1.8V to 3.0V logic, while TMP106
has 2.7V to 5.5V logic.
