Theory Of Operation - Clifton LAboratories Z10000?U Assembly And Operation Manual

2.2 Theory of Operation
2.2.1 Power Supply.
The AD8007 amplifier is rated at an absolute maximum operating voltage of 12V. In order to provide a
safety margin, and to decouple the amplifier from the power supply, U902, a three‐terminal fixed
regulator, provides a source of stable +9V to U901. C907 and C908 provide additional decoupling. U901,
the AD8007 amplifier, obtains its power via the RC decoupling network comprised of R902, C902 and
C906.
Since the circuit operates from a single positive power supply, it is necessary to bias U901's input to
approximately V/2. This is accomplished by the 2:1 voltage divider chain of R902/R903. C903 bypasses the
V/2 reference voltage; whilst R905 increases U901's input impedance by isolating C903's RF ground.
R905's maximum value is determined by the U901's input bias current on the positive pin, specified by
Analog Devices as 8μA. For 4.7 KΩ, this bias current represents an IR drop of 38 mV. With a DC gain of 4,
the corresponding output DC offset will be about 150 mV. If absolutely necessary for high input
impedance, R905 may be substituted with a higher value resistor, up to approximately 22 KΩ. However, at
frequencies above a few MHz, the input impedance is dominated by shunt capacitance; increasing R905
should be done only with an understanding of all the factors affecting the input impedance.
2.2.2 Amplifier
U901, an Analog Devices AD8007, is a high performance, low noise current feedback amplifier, with a
gain‐bandwidth product exceeding 650 MHz. A current feedback amplifier is also known as a
"transimpedance" amplifier. Analog Devices describes how a current feedback amplifier works:
First, the negative input of a CFA responds to current; the output voltage is proportional to that
current, hence transimpedance (V(out) = Z(t) I(in)). Instead of keeping the negative input current
small by maintaining high input impedance, and using feed‐back and voltage gain to keep the
input voltage difference small, the CFA keeps the voltage difference small by virtue of its low
input impedance (like looking back into a low‐offset emitter follower); and it keeps its net input
current small dynamically by feedback from the output.
When an ideal CFA is driven at the high‐impedance positive input, the negative input, with its low
impedance, follows closely in voltage; and the high gain for error current and the negative
feedback through Rf require that the currents through Rf and Rin be equal; hence V(out) =
V(in)[R(f)/R(in) + 1], just like for voltage‐feedback amplifiers. A major difference is that the slew
rate can be quite high, because large transient currents can flow in the input stage to handle
rapid changes in voltage across the compensating capacitor(s). Also, the low impedance at the
negative input means that stray input capacitance will not substantially affect the amplifier's
bandwidth.
U901's gain (in dB) is determined by the ratio of resistors R906 and R907:
3
Clifton Laboratories will provide interested Z10000 owners with the LTSpice model of the AD8007 and an
LTSpice model of the amplifier circuits upon request. SPICE modeling will allow the user to determine the effects of
component changes with a reasonable degree of accuracy.
Clifton Laboratories – Z10000‐U Buffer Amplifier Assembly and Operation
3
Page 5