Datasheet AD8362 (Analog Devices) - 19

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AD8362 Data Sheet
Accordingly, VTGT (and its fractional part VATG) determines In practice, the response deviates slightly from the ideal straight the output that must be provided by the VGA for the AGC loop line suggested by Equation 11. This deviation is called the law to settle. Because the scaling parameters of the two squarers are conformance error. In defining the performance of high accuracy accurately matched, it fol ows that Equation 4 is satisfied only when measurement devices, it is customary to provide plots of this MEAN(V 2 2 error. In general terms, it is computed by extracting the best SIG ) = VATG (5) straight line to the measured data using linear regression over In a formal solution, extract the square root of both sides to provide a substantial region of the dynamic range and under clearly an explicit value for the root-mean-square (rms) value. However, it specified conditions. is apparent that by forcing this identity through varying the VGA
3.0
gain and extracting the mean value by the filter provided by the
3.8 2.5
capacitor(s), the system inherently establishes the relationship
3.5 2.0 3.2 –40°C
rms(VSIG) = VATG (6)
1.5 2.9
Substituting the value of V
1.0
SIG from Equation 3,
2.6 (dB) 2.3 0.5 UT
rms[G
O
OVIN exp(−VSET/VGNS)] = VATG (7)
T (V) U 2.0 0 N V
As a measurement device, VIN is the unknown quantity and all
VO 1.7 –0.5 +25°C R I +85°C
other parameters can be fixed by design. To solve Equation 7,
1.4 –1.0 RRO 1.1 E –40°C
rms[G
–1.5
OVIN/VATG] = exp(VSET/VGNS) (8)
0.8 +25°C –2.0
therefore,
0.5 +85°C –2.5 0.2
VSET = VGNS log[rms(VIN)/VZ] (9)
–3.0 –60 –55 –50 –45 –40 –35 –30 –25 –20 –15 –10 –5 0 5 10 15
045 The quantity V
INPUT AMPLITUDE (dBm)
Z = VATG/GO is defined as the intercept voltage 02923- because VSET must be 0 when rms (V Figure 45. Output Voltage and Law Conformance Error IN) = VZ. at TA = −40°C, +25°C, and +85°C When connected as a measurement device, the output of the buffer is tied directly to VSET, which closes the AGC loop. Figure 45 shows the output of the circuit of Figure 47 over the Making the substitution VOUT = VSET and changing the full input range. The agreement with the ideal function (law log base to 10, as needed in a decibel conversion, conformance) is also shown. This was determined by linear regression on the data points over the central portion of the VOUT = VSLP log10[rms(VIN)/VZ] (10) transfer function for the +25°C data. where VSLP is the slope voltage, that is, the change in output The error at −40°C, +25°C, and +85°C was then calculated by voltage for each decade of change in the input amplitude. subtracting the ideal output voltage at each input signal level Note that VSLP = VGNS log (10) = 2.303 VGNS. from the actual output and dividing this quantity by the mean In the AD8362, VSLP is laser-trimmed to 1 V using a 100 MHz slope of the regression equation to provide a measurement of test signal. Because a decade corresponds to 20 dB, this slope the error in decibels (scaled on the right-hand axis of Figure 45). can also be stated as 50 mV/dB. The Altering the Slope section The error curves generated in this way reveal not only the devia- explains how the effective value of VSLP can be altered by the tions from the ideal transfer function at a nominal temperature, user. The intercept, VZ, is also laser-trimmed to 224 µV (−60 dBm but also the additional errors caused by temperature changes. relative to 50 Ω). In an ideal system, VOUT would cross zero Notice that there is a small temperature dependence in the for an rms input of that value. In a single-supply realization of intercept (the vertical position of the error plots). the function, VOUT cannot run fully down to ground; here, VZ is the extrapolated value. Figure 45 further reveals a periodic ripple in the conformance curves. This is due to the interpolation technique used to select
VOLTAGE vs. POWER CALIBRATION
the signals from the attenuator, not only at discrete tap points, The AD8362 can be used as an accurate rms voltmeter from but also anywhere in between, thus providing continuous arbitrarily low frequencies to microwave frequencies. For low attenuation values. The selected signal is then applied to the frequency operation, the input is usually specified either in 3.5 GHz, 40 dB fixed gain amplifier in the remaining stages of volts rms or in dBV (decibels relative to 1 V rms). the VGA of the AD8362. At high frequencies, signal levels are commonly specified in power terms. In these circumstances, the source and termina- tion impedances are an essential part of the overal scaling. For this condition, the output voltage can be expressed as VOUT = SLOPE × (PIN − PZ) (11) where PIN and the intercept PZ are expressed in dBm. Rev. F | Page 18 of 33 Document Outline FEATURES APPLICATIONS FUNCTIONAL BLOCK DIAGRAM GENERAL DESCRIPTION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS EQUIVALENT CIRCUITS TYPICAL PERFORMANCE CHARACTERISTICS CHARACTERIZATION SETUP EQUIPMENT ANALYSIS CIRCUIT DESCRIPTION SQUARE LAW DETECTION VOLTAGE vs. POWER CALIBRATION OFFSET ELIMINATION TIME-DOMAIN RESPONSE OF THE CLOSED LOOP OPERATION IN RF MEASUREMENT MODE BASIC CONNECTIONS DEVICE DISABLE RECOMMENDED INPUT COUPLING Choosing Input Coupling Capacitors Single-Ended Input Drive OPERATION AT LOW FREQUENCIES CHOOSING A VALUE FOR CHPF CHOOSING A VALUE FOR CLPF ADJUSTING VTGT TO ACCOMMODATE SIGNALS WITH VERY HIGH CREST FACTORS ALTERING THE SLOPE TEMPERATURE COMPENSATION AND REDUCTION OF TRANSFER FUNCTION RIPPLE TEMPERATURE COMPENSATION AT VARIOUS WiMAX FREQUENCIES UP TO 3.8 GHz OPERATION IN CONTROLLER MODE RMS VOLTMETER WITH 90 dB DYNAMIC RANGE AD8362 EVALUATION BOARD OUTLINE DIMENSIONS ORDERING GUIDE