LMP2021MAEVAL National Semiconductor, LMP2021MAEVAL Datasheet - Page 16

• Manufacturer Part Number: LMP2021MAEVAL
• Description: BOARD EVAL FOR LMP2021 OP AMP
• Manufacturer: National Semiconductor
• Series: LMP®r

Specifications of LMP2021MAEVAL

• Channels Per Ic: 1 - Single
• Amplifier Type: Instrumentation
• Output Type: Differential
• Slew Rate: 2.6 V/µs
• Current - Output / Channel: 50mA
• Operating Temperature: -40°C ~ 125°C
• Current - Supply (main Ic): 1.1mA
• Voltage - Supply, Single/dual (±): 2.2 V ~ 5.5 V
• Board Type: Fully Populated
• Utilized Ic / Part: LMP2021
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• -3db Bandwidth: -

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C
eliminating the reduction in A
An alternative circuit to achieve this is shown in Figure 11.
TRANSIENT RESPONSE TO FAST INPUTS
On chip continuous auto zero correction circuitry eliminates
the 1/f noise and significantly reduces the offset voltage and
offset voltage drift; all of which are very low frequency events.
For slow changing sensor signals this correction is transpar-
ent. For excitations which may otherwise cause the output to
swing faster than 40 mV/µs, there are additional considera-
tions which can be viewed two perspectives: for sine waves
and for steps.
For sinusoidal inputs, when the output is swinging rail-to-rail
on ±2.5V supplies, the auto zero circuitry will introduce dis-
tortions above 2.55 kHz. For smaller output swings, higher
frequencies can be amplified without the auto zero slew limi-
tation as shown in table below. Signals above 20 kHz, are not
affected, though normally, closed loop bandwidth should be
kept below 20 kHz so as to avoid aliasing from the auto zero
circuit.
For step-like inputs, such as those arising from disturbances
to a sensing system, the auto zero slew rate limitation mani-
fests itself as an extended ramping and settling time, lasting
~100 µs.
DIFFERENTIAL BRIDGE SENSOR
Bridge sensors are used in a variety of applications such as
pressure sensors and weigh scales. Bridge sensors typically
have a very small differential output signal. This very small
signal needs to be accurately amplified before it can be fed
into an ADC. As discussed in the previous sections, the ac-
curacy of the op amp used as the ADC driver is essential to
maintaining total system accuracy.
The high DC performance of the LMP2021/LMP2022 make
these amplifiers ideal choices for use with a bridge sensor.
The LMP2021/LMP2022 have very low input offset voltage
and very low input offset voltage drift. The open loop gain of
the LMP2021/LMP2022 is 160 dB.
IN
FIGURE 11. Alternative Sensor Impedance Circuit
in Figure 10 adds a zero to the low pass filter and hence
V
OUT-PEAK
0.32
2.5
1
(V)
VOL
f
of the LMP2021/LMP2022.
MAX-SINE WAVE
6.3
2.5
20
(kHz)
30014971
16
The on chip EMI rejection filters available on the LMP2021/
LMP2022 help remove the EMI interference introduced to the
signal and hence improve the overall system performance.
The circuit in Figure 12 shows a signal path solution for a typ-
ical bridge sensor using the LMP2021/LMP2022. Bridge sen-
sors are created by replacing at least one, and up to all four,
of the resistors in a typical bridge with a sensor whose resis-
tance varies in response to an external stimulus. Using four
sensors has the advantage of increasing output dynamic
range. Typical output voltage of one resistive pressure sensor
is 2 mV per 1V of bridge excitation voltage. Using four sen-
sors, the output of the bridge is 8 mV per 1V. The bridge
voltage is this system is chosen to be 1/2 of the analog supply
voltage and equal to the reference voltage of the AD-
C161S626, 2.5V. This excitation voltage results in 2.5V * 8
mV = 20 mV of differential output signal on the bridge. This
20 mV signal must be accurately amplified by the amplifier to
best match the dynamic input range of the ADC. This is done
by using one LMP2022 and one LMP2021 in front of the AD-
C161S626. The gaining of this 20 mV signal is achieved in 2
stages and through an instrumentation amplifier. The
LMP2022 in Figure 12 amplifies each side of the differential
output of the bridge sensor by a gain 18. Bridge sensor mea-
surements are usually done up to 10s of Hz. Placing a
300 Hz filter on the LMP2022 helps removing the higher fre-
quency noise from this circuit. This filter is created by placing
two capacitors in the feedback path of the LMP2022 ampli-
fiers. Using the LMP2022 with a gain of 18 reduces the input
referred voltage noise of the op amps and the system as a
result. Also, this gain allows direct filtering of the signal on the
LMP2022 without compromising noise performance. The dif-
ferential output of the two amplifiers in the LMP2022 are then
fed into a LMP2021 configured as a difference amplifier. This
stage has a gain of 5, with a total system having a gain of
(18*2+1)*5 = 185. The LMP2021 has an outstanding CMRR
value of 139. This impressive CMRR improves system per-
formance by removing the common mode signal introduced
by the bridge. With an overall gain of 185, the 20 mV differ-
ential input signal is gained up to 3.7V. This utilizes the
amplifiers output swing as well as the ADC's input dynamic
range.
This amplified signal is then fed into the ADC161S626. The
ADC161S626 is a 16-bit, 50 kSPS to 250 kSPS 5V ADC. In
order to utilize the maximum number of bits of the AD-
C161S626 in this configuration, a 2.5V reference voltage is
used. This 2.5V reference is also used to power the bridge
sensor and the inverting input of the ADC. Using the same
voltage source for these three points helps reducing the total
system error by eliminating error due to source variations.
With this system, the output signal of the bridge sensor which
can be up to 20 mV is accurately gained to the full scale of
the ADC and then digitized for further processing. The
LMP2021/LMP2022 introduced minimal error to the system
and improved the signal quality by removing common model
signals and high frequency noise.