AD834
BASIC OPERATION
Figure 7 is a functional equivalent of the AD834. There are
three differential signal interfaces: the voltage inputs X =
X1–X2 and Y = Y1–Y2, and the current output, W (see Figure
7) which flows in the direction shown when X and Y are posi-
tive. The outputs W1 and W2 each have a standing current of
typically 8.5 mA.
Figure 8. Basic Connections for Wideband Operation
impedance is quite high (about 25 kΩ), the input bias current of
typically 45 µA can generate significant offset voltages if not
compensated. For example, with a source and termination
resistance of 50 Ω (net source of 25 Ω) the offset would be
25 Ω × 45 µA = 1.125 mV. This can be almost fully cancelled by
including (in this example) another 25 Ω resistor in series with
the “unused” input (in Figure 8, either X1 or Y2). In order to
minimize crosstalk the input pins closest to the output (X1 and
Y2) should be grounded; the effect is merely to reverse the
phase of the X input and thus alter the polarity of the output.
Figure 7. AD834 Functional Block Diagram
The input voltages are first converted to differential currents
which drive the translinear core. The equivalent resistance of
the voltage-to-current (V-I) converters is about 285 Ω. This low
value results in low input related noise and drift. However, the
low full-scale input voltage results in relatively high nonlinearity
in the V-I converters. This is significantly reduced by the use of
distortion cancellation circuits which operate by Kelvin sensing
the voltages generated in the core—an important feature of the
AD834.
TRANSFER FUNCTION
The output current W is the linear product of input voltages
X and Y divided by (1 V)2 and multiplied by the “scaling
current” of 4 mA:
The current mode output of the core is amplified by a special
cascode stage which provides a current gain of nominally × 1.6,
trimmed during manufacture to set up the full-scale output cur-
rent of 4 mA. This output appears at a pair of open
collectors which must be supplied with a voltage slightly
above the voltage on Pin 6. As shown in Figure 8, this can be
arranged by inserting a resistor in series with the supply to this
pin and taking the load resistors to the full supply. With R3 =
60 Ω, the voltage drop across it is about 600 mV. Using two
50 Ω load resistors, the full-scale differential output voltage is
400 mV.
XY
W =
4mA
2
1V
(
)
Provided that it is understood that the inputs are specified in
volts, a simplified expression can be used:
W =(XY )4mA
Alternatively, the full transfer function can be written:
XY
1V 250 Ω
1
W =
×
The full bandwidth potential of the AD834 can only be realized
when very careful attention is paid to grounding and decou-
pling. The device must be mounted close to a high quality
ground plane and all lead lengths must be extremely short, in
keeping with UHF circuit layout practice. In fact, the AD834
shows useful response to well beyond 1 GHz, and the actual up-
per frequency in a typical application will usually be determined
by the care with which the layout is effected. Note that R4 (in
series with the –VS supply) carries about 30 mA and thus intro-
duces a voltage drop of about 150 mV. It is made large enough
to reduce the Q of the resonant circuit formed by the supply
lead and the decoupling capacitor. Slightly larger values can be
used, particularly when using higher supply voltages. Alterna-
tively, lossy RF chokes or ferrite beads on the supply leads may
be used.
When both inputs are driven to their clipping level of about
1.3 V, the peak output current is roughly doubled, to 8 mA,
but distortion levels will then be very high.
TRANSFORMER COUPLING
In many high frequency applications where baseband operation
is not required at either inputs or output, transformer coupling
can be used. Figure 9 shows the use of a center-tapped output
transformer, which provides the necessary dc load condition at
the outputs W1 and W2, and is designed to match into the de-
sired load impedance by appropriate choice of turns ratio. The
specific choice of the transformer design will depend entirely on
the application. Transformers may also be used at the inputs.
Center-tapped transformers can reduce high frequency distor-
tion and lower HF feedthrough by driving the inputs with
balanced signals.
Figure 8 shows the use of optional termination resistors at the
inputs. Note that although the resistive component of the input
REV. C
–5–
5秒后页面跳转
PCF8574资料解读：主要参数分析、引脚说明
AD637数据手册解读：主要特性、引脚及其功能解读、电气参数
ADUM1201资料手册解读：参数分析、引脚说明、应用分析
一文带你了解压敏电阻器在直流电路中的过压保护作用
微信公众号
抖音公众号
浙公网安备 33010502006866号 浙ICP备10014259号-119
营业执照ICP证
技术参数
数据手册
如果您发现信息不准确，
