LMV841MG National Semiconductor Corporation, LMV841MG Datasheet - Page 14

• Manufacturer Part Number: LMV841MG
• Description: The ADC14V155 is a high-performance CMOS analog-to-digital converter with LVDS outputs. It is capable of converting analog input signals into 14-Bit digital words at rates up to 155 Mega Samples Per Second (MSPS). Data leaves the chip in a DDR (Dual
• Manufacturer: National Semiconductor Corporation

www.national.com
The filter responses of filter A and filter B are shown as the
thin lines in Figure 4, the response of the combined filter is
shown as the thick line. Shifting the center frequencies of the
separate filters farther apart, will result in a wider band, how-
ever positioning the center frequencies too far apart will result
in a less flat gain within the band. For wider bands more band-
pass filters can be cascaded.
Tip: Use the WEBENCH internet tools at www.national.com
for your filter application.
HIGH-SIDE CURRENT SENSING
The rail-to-rail input and the low V
LMV841/844 ideal op amps for high-side current sensing ap-
plication.
To measure a current, a sense resistor is placed in series with
the load, as shown in Figure 5. The current flowing through
this sense resistor will result in a voltage drop, that is amplified
by the op amp.
Suppose we need to measure a current between 0A and 2A
using a sense resistor of 100 mΩ, and convert it to an output
voltage of 0 to 5V. A current of 2A flowing through the load
and the sense resistor will result in a voltage of 200 mV across
the sense resistor. The op amp will amplify this 200 mV to fit
the current range to the output voltage range. We can use the
formula:
V
to calculate the gain needed. For a load current of 2A and an
output voltage of 5V the gain would be V
When we use a feedback resistor, R
R
low to obtain a good common-mode rejection.
HIGH IMPEDANCE SENSOR INTERFACE
With CMOS inputs, the LMV841/LMV844 are particularly suit-
ed to be used as high impedance sensor interfaces.
Many sensors have high source impedances that may range
up to 10 MΩ. The input bias current of an amplifier will load
the output of the sensor, and thus cause a voltage drop across
the source resistance, as shown in Figure 6. When an op amp
is selected with a relatively high input bias current, this error
may be unacceptable.
The low input current of the LMV841/LMV844 significantly re-
duces such errors. The following examples show the differ-
ence between a standard op amp input and the CMOS input
of the LMV841/LMV844.
OUT
G
would be 4 kΩ. The tolerance of the resistors has to be
= R
F
FIGURE 5. High-Side Current Sensing
/ R
G
* V
SENSE
F
, of 100 kΩ the value for
OS
OUT
features make the
/ V
SENSE
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= 25.
14
The voltage at the input of the op amp can be calculated by
V
For a standard op amp the input bias Ib could be 10 nA. When
the sensor generates a signal of 1V (V
impedance is 10 MΩ (R
be
V
For the CMOS input of the LMV841/LMV844, which has an
input bias current of only 0.3 pA, this would give
V
The conclusion is that a standard op amp, with its high input
bias current input, is not a good choice for use in impedance
sensor applications. The LMV841/LMV844, in contrast, are
much more suitable due to the low input bias current. The
error is negligibly small, therefore the LMV841/LMV844 are a
must for use with high impedance sensors.
THERMOCOUPLE AMPLIFIER
The following is a typical example for a thermocouple ampli-
fier application with an LMV841/LMV844. A thermocouple
senses a temperature and converts it into a voltage. This sig-
nal is then amplified by the LMV841. An ADC can then convert
the amplified signal to a digital signal. For further processing
the digital signal can be processed by a microprocessor and
can be used to display or log the temperature, or use the
temperature data in a fabrication process.
Characteristics of a Thermocouple
A thermocouple is a junction of two different metals. These
metals produce a small voltage that increases with tempera-
ture.
The thermocouple used in this application is a K-type ther-
mocouple. A K-type thermocouple is a junction between Nick-
el-Chromium and Nickel-Aluminum. This type is one of the
most commonly used thermocouples. There are several rea-
sons for using the K-type thermocouple. These include tem-
perature range, the linearity, the sensitivity and the cost.
A K-type thermocouple has a wide temperature range. The
range of this thermocouple is from approximately −200°C to
approximately 1200°C, as can be seen in Figure 7. This cov-
ers the generally used temperature ranges.
Over the main part of the range the behavior is linear. This is
important for converting the analog signal to a digital signal.
The K-type thermocouple has good sensitivity when com-
pared to many other types, the sensitivity is 41 uV/°C. Lower
sensitivity requires more gain and makes the application more
sensitive to noise.
In addition, a K-type thermocouple is not expensive, many
other thermocouples consist of more expensive materials or
are more difficult to produce.
IN+
IN
IN
= 1V - 10 nA * 10 MΩ = 1V - 0.1V = 0.9V
= 1V – 0.3 pA * 10 MΩ = 1V - 3 μV = 0.999997 V !
= V
FIGURE 6. High Impedance Sensor Interface
S
- I
B
* R
S
S
), the signal at the op amp input will
S
) and the sensors
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