HMC714LP5 Datasheet, PDF (38/40 Pages) Hittite Microwave Corporation – DUAL RMS POWER DETECTOR 0.1 - 3.9 GHz

v05.0309
HMC714LP5 / 714LP5E
DUAL RMS POWER DETECTOR
0.1 - 3.9 GHz
DC Offset Compensation Loop (Continued)
DC Offset Cancellation Loop Bandwith _≈
1
Hz
π(500)(C + 20 x 10¹²)
OFS
For example: loop bandwidth for DC cancellation with COFS = 1nF, bandwidth is ~62 kHz
Note: The measurement error produced by internal DC offsets cannot be measured repeatably at any single operating
point, in terms of input signal frequency and level. Measurement error must be calculated to a best fit line, over the
entire range of input signal (again, in terms of signal level and frequency).
Adjusting V_TGT for Greater Precision
There are two competing aspects of performance, for which V_TGT can be used to set a preference. Depending on
which aspect of precision is more important to the application, the V_TGT pin can be used to find a compromise between
two sources of RMS output error: internal DC offset cancellation error and deviation at high crest factors (>12dB).
• Increasing V_TGT input voltage will reduce the effect of internal DC offsets, but deviation at high crest factors
will increase slightly. A 50% increase in V_TGT should produce an 18% improvement in RMS precision due to a
reduction in internal DC offsets effects.
• Decreasing V_TGT input voltage will reduce errors at high crest factors, but internal DC offsets will have more
of an effect on measurement accuracy.
If input signal crest factor is not expected to exceed 10dB, you can improve RMS precision by increasing V_TGT voltage.
Keep in mind that changing V_TGT also adjusts the log-intercept point, which shifts the “input dynamic range”. The best
set-point for V_TGT will be the lowest voltage that still maintains the “input dynamic range” over the required range of
input power. This new V_TGT set-point should optimize the amount of DC offset related errors.
If error performance at high crest factors requires optimization, set V_TGT for the maximum tolerable error at the
highest expected crest factor. Increasing V_TGT beyond that point will unnecessarily compromise internal DC offset
cancellation performance. After changing V_TGT, re-verify that the “input dynamic range” still covers the required range
of input power.
V_TGT should be referenced to V_REF for best performance. It is recommended to use a temperature stable DC amplifier
between V_TGT and V_REF to create V_TGT > V_REF. The V_REF pin is a temperature compensated voltage reference output,
only intended for use with V_TGT.
12
RMS Output Error vs. Crest Factor
0
-0.5
-1
-1.5
-2
VTGT=0.5V
VTGT=1V
-2.5
VTGT=2V
VTGT=3V
-3
0
2
4
6
8
10
12
14
CREST FACTOR (dB)
V_TGT influence on DC offset compensation
V
GTG
Error due to
internal DC offsets
1.0 V
nominal + 0.2 dB
1.5 V
nominal + 0.1 dB
2.0 V
nominal
3.0 V
7.nominal + 0.06 dB
3.5 V
nominal + 0.1 dB
For price, delivery, and to place orders, please contact Hittite Microwave Corporation:
20 Alpha Road, Chelmsford, MA 01824 Phone: [phone] Fax: [phone]
Order On-line at www.hittite.com
12 - 151