Texas Instruments Single 8 MHz Low Power CMOS, EMI Hardened Operational Amplifier, Evaluation module LMV851EVAL/NOPB LMV851EVAL/NOPB Datenbogen
Produktcode
LMV851EVAL/NOPB
RF INPUT PEAK VOLTAGE (dBVp)
´
V
O
S
(d
BV
)
-20
-40
-60
-80
-100
-120
-40
-30
-20
-10
0
10
2
1
¸
¸
¹
·
¨
¨
©
§
'
V
OS
V
RF_PEAK
EMIRR
V
RF_PEAK
= 20 log
RF SIGNAL
V
OUT OPAMP
(A
V
= 1)
NO RF
RF
V
OS
+ V
DETECTED
V
OS
EMIRR Definition
Figure 1. Offset Voltage Variation Due to a Detected RF Signal
3
EMIRR Definition
To identify EMI robust op amps, a parameter is needed that quantitatively describes the EMI performance.
A quantitative measure enables the comparison and the ranking of op amps on their EMI robustness. This
application report introduces the EMI Rejection Ratio (EMIRR). This parameter describes the resulting
input-referred offset voltage shift of an op amp as a result of an applied RF carrier (interference) with a
certain frequency and level. The definition of EMIRR is given by:
A quantitative measure enables the comparison and the ranking of op amps on their EMI robustness. This
application report introduces the EMI Rejection Ratio (EMIRR). This parameter describes the resulting
input-referred offset voltage shift of an op amp as a result of an applied RF carrier (interference) with a
certain frequency and level. The definition of EMIRR is given by:
(1)
where V
RF_PEAK
is the amplitude of the applied unmodulated RF signal (V) and
Δ
V
OS
is the resulting input-
referred offset voltage shift (V).
In this definition, the RF signal level is included as a condition at which the EMIRR is determined. This is
required as the relation between the resulting offset voltage shift and the RF signal level is quadratic (the
details on this quadratic relation is beyond the scope of this application report). An example of the
resulting offset shift (
required as the relation between the resulting offset voltage shift and the RF signal level is quadratic (the
details on this quadratic relation is beyond the scope of this application report). An example of the
resulting offset shift (
Δ
V
OS
) versus applied RF level (RF peak voltage) is shown in
.
describes in more detail the measurement setup used for obtaining these results). The curve is shown on
a LOG-LOG scale to clearly show the quadratic nature of the offset voltage shift versus RF level, that is,
the curve has a slope of two. The curve is limited at the bottom end of the signal range as for the
corresponding relatively low RF signal levels the resulting offset shift is below the resolution of the
measurement setup (noise). For op amps with a relatively high sensitivity (low EMIRR), the curve might
saturate for higher RF input levels. This is a result of the offset shift that becomes very large, such that the
op amp clips.
a LOG-LOG scale to clearly show the quadratic nature of the offset voltage shift versus RF level, that is,
the curve has a slope of two. The curve is limited at the bottom end of the signal range as for the
corresponding relatively low RF signal levels the resulting offset shift is below the resolution of the
measurement setup (noise). For op amps with a relatively high sensitivity (low EMIRR), the curve might
saturate for higher RF input levels. This is a result of the offset shift that becomes very large, such that the
op amp clips.
Figure 2. Measured Input Referred Offset Voltage Shift vs. Applied RF Peak Level
3
SNOA497B – September 2007 – Revised April 2013
AN-1698 A Specification for EMI Hardened Operational Amplifiers
Copyright © 2007–2013, Texas Instruments Incorporated