Datasheet AD8307 (Analog Devices) - 21

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AD8307 Data Sheet APPLICATIONS INFORMATION
The AD8307 is a highly versatile and easily applied log amp
0.1µF 4.7Ω V
requiring very few external components. Most applications of this
P, 2.7V TO 5.5V VR2
device can be accommodated using the simple connections shown
50kΩ RS
in the preceding section.
±3dB 2N3904 8 7 6 5 BUFFERED OUTPUT INPUT INP VPS ENB INT 25mV/dB –75dBm TO AD8031 +16dBm AD8307
The output can be buffered and the slope optionally increased by
RT INM COM OFS OUT (OPTIONAL)
using an op amp. If the single-supply capability is to be preserved, a
1 2 3 4 10mV/dB R2 ±18% 3.01kΩ OUTPUT
suitable component is the AD8031. Like the AD8307, it is capable
NC 50Ω MINIMUM
of operating from a 2.7 V supply and features a rail-to-rail
NC = NO CONNECT 6.34kΩ VR1 R1
-039 output capability; it is available in a 5-lead version and in dual
5kΩ 2kΩ
082 01
COM
form as the 8-lead AD8032. Figure 38 shows how the slope can Figure 39. Cable Driving Log Amp be increased to 50 mV/dB (1 V per decade), requiring a 5 V supply (90 dB times 50 mV is a 4.5 V swing). VR1 provides a In Figure 40, the capacitor values are chosen for operation in the ±10% slope adjustment; VR2 provides a ±3 dB intercept range. audio field, providing a corner frequency of 10 Hz, an attenuation With R2 = 4.99 kΩ, the slope is adjustable to 25 mV/dB, allowing of 80 dB/decade above this frequency, and a 1% settling time of the use of a 2.7 V supply. Setting R2 to 80.6 kΩ, it is raised to 150 ms (0.1% in 175 ms). The residual ripple is 4 mV (±0.02 dB) 100 mV/dB, providing direct reading in decibels on a digital when the input to the AD8307 is at 20 Hz. This filter can easily voltmeter. Because a 90 dB range now corresponds to a 9 V swing, be adapted to other frequencies by proportional scaling of C5 to C7 a supply of at least this amount is needed for the op amp. (for example, for 100 kHz use 100 pF). Placed ahead of a digital
0.1µF
multimeter, the convenient slope scaling of 100 mV/dB requires
4.7Ω VP, 2.7V TO 5.5V
only a repositioning of the decimal point to read directly in
VR2 50kΩ RS
decibels. The supply voltage for the filter must be large enough to support the dynamic range; a minimum of 9 V is needed for most
±3dB FOR VP = 3V, RS = 20kΩ 8 7 6 5 V
applications; 12 V is recommended.
P = 5V, RS = 51kΩ INPUT INP VPS ENB INT OUTPUT –75dBm TO AD8031 INPUT 5mV 50mV/dB 0.1µF +16dBm AD8307 TO 160V rms ±10% 4.7Ω V INM COM OFS OUT P OP AMP IS AD8032 SCALE R1 VR1 C1 TO C8 AS NEEDED. 1 2 3 4 R2 20mV/dB 30.1kΩ 50kΩ 2kΩ 422Ω NOTE POLARITIES IF TANTALUM NC CAPACITORS ARE USED. C1 INT ±4dB 10µF + C5 C8 NC = NO CONNECT OUTPUT
38
32.4kΩ VR1 R1 1µF 1µF
0
C1 7.32kΩ 100mV/dB
2-
50kΩ 20kΩ NC
08
COM 8 7 6 5 + +
01
INP VPS ENB INT
Figure 38. Log Amp with Buffered Output
100kΩ C3 AD8307 93kΩ 2.5nF
C1 is optional; it lowers the corner frequency of the low-pass
INM COM OFS OUT 1 2 3 4 34kΩ
output filter. A value of 0.1 μF should be used for applications in
34kΩ +
which the output is measured on a voltmeter or other low speed
VR2 C7 F + 50kΩ 1µF + C4 1µ 75kΩ
device. On the other hand, when C1 is omitted, the 10% to 90%
C2 SLOPE C6+
0
10µF 32.4kΩ 80.6kΩ
04
1µF
response time is under 200 ns and is typically 300 ns to 99% of 2- 08
COM
01 the final value. To achieve faster response times, it is necessary Figure 40. Log Amp with Four-Pole Low-Pass Filter to lower the load resistance at the output of the AD8307, then Figure 40 also shows the use of an input attenuator that can restore the scale using a higher gain in the op amp. Using 8.33 kΩ, optionally be employed to produce a useful wide range ac voltmeter the basic slope is 10 mV/dB; this can be restored to 25 mV/dB with direct decibel scaling. The basic range of −73 dBm to using a buffer gain of 2.5. The overall 10% to 90% response time +17 dBm (that is, 50 μV rms to 1.6 V rms, for sine excitations) is is under 100 ns. Figure 39 shows how the output current capability shifted for illustrative purposes to 5 mV to 160 V rms (at which can be augmented to drive a 50 Ω load; RT optionally provides point the power in R1 is 512 mW). Because the basic input reverse termination, which halves the slope to 12.5 mV/dB. resistance of the AD8307 is not precise, VR1 is used to center the
FOUR-POLE FILTER
signal range at its input, doubling as a ±4 dB intercept adjustment. In low frequency applications, for example, audio down to The low frequency response extends to 15 Hz; a higher corner 20 Hz, it is useful to employ the buffer amplifier as a multipole frequency can be selected as needed by scaling C1 and C2. The low-pass filter to achieve low output ripple while maintaining a shunt capacitor, C3, is used to lower the high frequency bandwidth rapid response time to changes in signal level. to about 100 kHz, and thus lower the susceptibility to spurious signals. Other values should be chosen as needed for the coupling and filter capacitors. Rev. E | Page 20 of 24 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 TYPICAL PERFORMANCE CHARACTERISTICS LOG AMP THEORY PROGRESSIVE COMPRESSION DEMODULATING LOG AMPS INTERCEPT CALIBRATION OFFSET CONTROL EXTENSION OF RANGE INTERFACES ENABLE INTERFACE INPUT INTERFACE OFFSET INTERFACE OUTPUT INTERFACE THEORY OF OPERATION BASIC CONNECTIONS INPUT MATCHING NARROW-BAND MATCHING SLOPE AND INTERCEPT ADJUSTMENTS APPLICATIONS INFORMATION BUFFERED OUTPUT FOUR-POLE FILTER 1 µW TO 1 kW 50 Ω POWER METER MEASUREMENT SYSTEM WITH 120 dB DYNAMIC RANGE OPERATION AT LOW FREQUENCIES OUTLINE DIMENSIONS ORDERING GUIDE