参数资料
| 型号： | AD8307ARZ-RL7 |
| 厂商： | Analog Devices Inc |
| 文件页数： | 2/24页 |
| 文件大小： | 0K |
| 描述： | IC AMP LOGARITHMIC 8-SOIC |
| 标准包装： | 1 |
| 类型： | 对数放大器 |
| 应用： | 接收器信号强度指示（RSSI） |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 8-SOIC（0.154"，3.90mm 宽） |
| 供应商设备封装： | 8-SO |
| 包装： | 标准包装 |
| 其它名称： | AD8307ARZ-RL7DKR |
AD8307
Rev. D | Page 10 of 24
just dB. The logarithmic function disappears from the formula
because the conversion has already been implicitly performed
in stating the input in decibels. This is strictly a concession to
popular convention; log amps manifestly do not respond to power
(tacitly, power absorbed at the input), but rather to input voltage.
The use of dBV (decibels with respect to 1 V rms) is more precise,
though still incomplete, because waveform is involved as well.
Because most users think about and specify RF signals in terms
of power, more specifically, in dBm re: 50 Ω, this convention is
used in specifying the performance of the AD8307.
PROGRESSIVE COMPRESSION
Most high speed, high dynamic range log amps use a cascade of
nonlinear amplifier cells (see Figure 22) to generate the logarithmic
function from a series of contiguous segments, a type of piecewise
linear technique. This basic topology immediately opens up the
possibility of enormous gain bandwidth products. For example,
the AD8307 employs six cells in its main signal path, each having
a small signal gain of 14.3 dB (×5.2) and a 3 dB bandwidth of
about 900 MHz. The overall gain is about 20,000 (86 dB) and
the overall bandwidth of the chain is some 500 MHz, resulting
in the incredible gain bandwidth product (GBW) of 10,000 GHz,
about a million times that of a typical op amp. This very high
GBW is an essential prerequisite for accurate operation under
small signal conditions and at high frequencies. In Equation 2,
however, the incremental gain decreases rapidly as VIN increases.
The AD8307 continues to exhibit an essentially logarithmic
response down to inputs as small as 50 μV at 500 MHz.
VX
VW
STAGE 1
STAGE 2
STAGE N–1
STAGE N
A
0
10
82
-0
22
Figure 22. Cascade of Nonlinear Gain Cells
To develop the theory, first consider a scheme slightly different
from that employed in the AD8307, but simpler to explain and
mathematically more straightforward to analyze. This approach
is based on a nonlinear amplifier unit, called an A/1 cell, with
the transfer characteristic shown in Figure 23.
The local small signal gain δVOUT/δVIN is A, maintained for all
inputs up to the knee voltage EK, above which the incremental
gain drops to unity. The function is symmetrical: the same drop
in gain occurs for instantaneous values of VIN less than –EK. The
large signal gain has a value of A for inputs in the range EK ≤
VIN ≤ +EK, but falls asymptotically toward unity for very large
inputs. In logarithmic amplifiers based on this amplifier function,
both the slope voltage and the intercept voltage must be traceable
to the one reference voltage, EK. Therefore, in this fundamental
analysis, the calibration accuracy of the log amp is dependent
solely on this voltage. In practice, it is possible to separate the
basic references used to determine VY and VX and, in the case of
the AD8307, VY is traceable to an on-chip band gap reference,
whereas VX is derived from the thermal voltage kT/q and is later
temperature corrected.
0
10
82
-02
3
SLOPE = A
SLOPE = 1
OU
TP
U
T AEK
EK
0
INPUT
A/1
Figure 23. A/1 Amplifier Function
Let the input of an N-cell cascade be VIN, and the final output
be VOUT. For small signals, the overall gain is simply AN. A
six-stage system in which A = 5 (14 dB) has an overall gain
of 15,625 (84 dB). The importance of a very high small signal
gain in implementing the logarithmic function has been noted;
however, this parameter is only of incidental interest in the design
of log amps.
From this point forward, rather than considering gain, analyze
the overall nonlinear behavior of the cascade in response to a
simple dc input, corresponding to the VIN of Equation 1. For
very small inputs, the output from the first cell is V1 = AVIN.
The output from the second cell is V2 = A2 VIN, and so on, up to
VN = AN VIN. At a certain value of VIN, the input to the Nth cell,
VN 1, is exactly equal to the knee voltage EK. Thus, VOUT = AEK
and because there are N 1 cells of Gain A ahead of this node,
calculate VIN = EK /AN 1. This unique situation corresponds to
the lin-log transition (labeled 1 in Figure 24). Below this input,
the cascade of gain cells acts as a simple linear amplifier, whereas
for higher values of VIN, it enters into a series of segments that
lie on a logarithmic approximation (dotted line).
RATIO
OF A
2
1
3
2
EK/AN–1 EK/AN–2 EK/AN–3 EK/AN–4
LOG VIN
(4A–3) EK
VOUT
(3A–2) EK
(2A–1) EK
AEK
0
(A–1) EK
01
08
2-
0
24
Figure 24. First Three Transitions
Continuing this analysis, the next transition occurs when the
input to the N 1 stage just reaches EK, that is, when VIN =
EK/AN 2. The output of this stage is then exactly AEK, and it is
easily demonstrated (from the function shown in Figure 23) that
Thus, the output has changed by an amount (A 1)EK for a
change in VIN from EK/AN1 to EK/AN2, that is, a ratio change of A.
At the next critical point (labeled 3 in Figure 24), the input is
again A times larger and VOUT has increased to (3A 2)EK, that
is, by another linear increment of (A 1)EK.
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