Texas Instruments Single 8 MHz Low Power CMOS, EMI Hardened Operational Amplifier, Evaluation module LMV851EVAL/NOPB LMV851EVAL/NOPB 데이터 시트
제품 코드
LMV851EVAL/NOPB
+
-
R
1
50
:
RFin
C
1
22 pF
C
2
10 µF
100 pF
V
DD
V
SS
OUT
+
C
3
100 pF
10 µF
+
C
4
C
5
EMIRR Measurement
4.4.2
Test #1: Coupling an RF Signal to the IN+ Pin
For testing the IN+ pin the op amp is connected in the unity gain configuration. Applying the RF signal is
straightforward, as it can be connected directly to the IN+ pin. As a result, there are a minimum of
disturbing components in the RF signal path. The circuit diagram is shown in
straightforward, as it can be connected directly to the IN+ pin. As a result, there are a minimum of
disturbing components in the RF signal path. The circuit diagram is shown in
. The PCB trace
from RFin to the IN+ pin should be a 50
Ω
stripline in order to match the RF impedance of the cabling and
the RF generator. On the PCB a 50
Ω
termination is used. This 50
Ω
resistor is also used to set the bias
level of the IN+ pin to ground level. The DC measurements are taken at the output of the op amp. As the
op amp is in the unity gain configuration, the input referred offset voltage shift corresponds one-to-one to
the measured output voltage shift.
op amp is in the unity gain configuration, the input referred offset voltage shift corresponds one-to-one to
the measured output voltage shift.
Figure 3. Coupling the RF Signal to the IN+ Pin Circuit Diagram
4.4.3
Test #2: Coupling an RF Signal to the IN
−
Pin
For coupling an RF signal to the IN
−
pin, the unity gain configuration as described for the IN+ pin cannot
be used. In that configuration the RF signal would be applied not only to the IN
−
pin but to the output pin
as well. For accurately measuring the EMIRR of the IN
−
pin, RF isolation is required for the output pin.
Therefore, a voltage gain configuration is used as shown in
. The low-frequency gain, which
applies to the resulting offset shift, is set to 2. The feedback resistor R
3
and the load capacitance isolate
the output pin from the injected RF signal at the IN
−
pin.
The gain of the feedback network is not important for the applied RF signal. Since the RF frequency is
much higher than the Gain-Bandwidth-Product (GBP) of the op amp, the op amp configuration can be
seen for RF signals as an open loop circuit. The gain of the op amp configuration is important for
translating the obtained output voltage shift to an input referred offset voltage shift. The input referred
offset voltage shift is calculated by dividing the measured output voltage shift by the voltage gain, which is
2 in this case.
much higher than the Gain-Bandwidth-Product (GBP) of the op amp, the op amp configuration can be
seen for RF signals as an open loop circuit. The gain of the op amp configuration is important for
translating the obtained output voltage shift to an input referred offset voltage shift. The input referred
offset voltage shift is calculated by dividing the measured output voltage shift by the voltage gain, which is
2 in this case.
Also for this PCB the signal path from RFin to the IN
−
pin should be a 50
Ω
stripline with appropriate
termination. The RF signal is applied to the IN
−
pin via a coupling capacitor C
1
. The parasitic series
inductance of this capacitor needs to be compared to the 50
Ω
impedance of the RF signal path. So, an
inductance of a few nH is acceptable when measuring up to a few GHz. As a result a standard SMD
component can be used here.
component can be used here.
For symmetry reasons it is expected that the positive and negative input have the same sensitivity for
applied RF signals but with an opposite polarity for the obtained input referred offset shift. The input pins
thus have the same EMIRR.
applied RF signals but with an opposite polarity for the obtained input referred offset shift. The input pins
thus have the same EMIRR.
6
AN-1698 A Specification for EMI Hardened Operational Amplifiers
SNOA497B – September 2007 – Revised April 2013
Copyright © 2007–2013, Texas Instruments Incorporated