AD8137YCPZ-R2 datasheet(26/32 Pages) AAVID

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AD8137

Data Sheet

Rev. E | Page 26 of 32

Estimating DC Errors

Primary differential output offset errors in the AD8137 are due

to three major components: the input offset voltage, the offset

between the VAN and VAP input currents interacting with the

feedback network resistances, and the offset produced by the

dc voltage difference between the input and output common-

mode voltages in conjunction with matching errors in the

feedback network.

The first output error component is calculated as













Vo_e1

, or equivalently as VIO/β

(21)

where VIO is the input offset voltage.

The second error is calculated as

( )

Vo_e

























(22)

where IIO is defined as the offset between the two input bias

currents.

The third error voltage is calculated as

Vo_e3 = Δenr × (VICM − VOCM)

(23)

where Δenr is the fractional mismatch between the two feedback

resistors.

The total differential offset error is the sum of these three error

sources.

Additional Impact of Mismatches in the Feedback Networks

The internal common-mode feedback network still forces the

output voltages to remain balanced, even when the RF/RG feed-

back networks are mismatched. The mismatch, however, causes

a gain error proportional to the feedback network mismatch.

Ratio-matching errors in the external resistors degrade the

ability to reject common-mode signals at the VAN and VIN input

terminals, similar to a four resistor, difference amplifier made

from a conventional op amp. Ratio-matching errors also produce a

differential output component that is equal to the VOCM input

voltage times the difference between the feedback factors (βs).

In most applications using 1% resistors, this component amounts

to a differential dc offset at the output that is small enough to

be ignored.

Driving a Capacitive Load

A purely capacitive load reacts with the bondwire and pin

inductance of the AD8137, resulting in high frequency ringing

in the transient response and loss of phase margin. One way to

minimize this effect is to place a small resistor in series with

each output to buffer the load capacitance. The resistor and load

capacitance forms a first-order, low-pass filter; therefore, the

resistor value should be as small as possible. In some cases, the

ADCs require small series resistors to be added on their inputs.

Figure 37 and Figure 40 illustrate transient response vs. capacitive

load and were generated using series resistors in each output

and a differential capacitive load.

Layout Considerations

Standard high speed PCB layout practices should be adhered

to when designing with the AD8137. A solid ground plane is

recommended and good wideband power supply decoupling

networks should be placed as close as possible to the supply pins.

To minimize stray capacitance at the summing nodes, the

copper in all layers under all traces and pads that connect to

the summing nodes should be removed. Small amounts of stray

summing-node capacitance cause peaking in the frequency

response, and large amounts can cause instability. If some stray

summing-node capacitance is unavoidable, its effects can be

compensated for by placing small capacitors across the feedback

resistors.

Terminating a Single-Ended Input

Controlled impedance interconnections are used in most high

speed signal applications, and they require at least one line

termination. In analog applications, a matched resistive termination

is generally placed at the load end of the line. This section deals

with how to properly terminate a single-ended input to the AD8137.

The input resistance presented by the AD8137 input circuitry

is seen in parallel with the termination resistor, and its loading

effect must be taken into account. The Thevenin equivalent

circuit of the driver, its source resistance, and the termination

resistance must all be included in the calculation as well. An

exact solution to the problem requires solution of several

simultaneous algebraic equations and is beyond the scope of

this data sheet. An iterative solution is also possible and is easier,

especially considering the fact that standard resistor values are

generally used.

AD8137YCPZ-R2 数据表(PDF) 26 Page - Aavid, Thermal Division of Boyd Corporation

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