HFA1120 Datasheet, PDF (15/17 Pages) Intersil Corporation – 850MHz Current Feedback Amplifier with Offset Adjust

HFA1120
DESIGN INFORMATION (Continued)
The information contained in this section has been developed through characterization by Intersil Semiconductor and is for use as
application and design information only. No guarantee is implied.
The layout and schematic of the board are shown in Figure
2. To order evaluation boards, please contact your local
sales office.
Offset Adjustment
The output offset voltage of the HFA1120 may be nulled via
connections to the BAL pins. Unlike a voltage feedback
amplifier, offset adjustment is accomplished by varying the
sign and/or magnitude of the inverting input bias current
(-I_BIAS). With voltage feedback amplifiers, bias currents are
matched and bias current induced offset errors are nulled by
matching the impedances seen at the positive and negative
inputs. Bias currents are uncorrelated on current feedback
amplifiers, so this technique is inappropriate.
TOP LAYOUT
V_H
1
+IN
OUT V+
V_{L V-}
GND
BOTTOM LAYOUT
-I_BIAS flows through R_F causing an output offset error.
Likewise, any change in -I_BIAS forces a corresponding
change in output voltage, providing the capability for output
offset adjustment. By nulling -I_BIAS to zero, the offset error
due to this current is eliminated. In addition, an adjustment
limit greater than the -I_BIAS limit allows the user to null the
contributions from other error sources, such as V_IO, or +IN
source impedance. For example, the excess adjust current
of 50µA (I_BNADJ min. - I_BSN max.) allows for the nulling of an
additional 26mV of output offset error (with R_F = 510Ω) at
room temperature. The amount of adjustment is a function of
500
R_F , so adjust range increases with increased R_F . If allowed
by other considerations, such as bandwidth and noise, R_F
can be increased to provide more adjustment range.
R1
1
50Ω
2
The recommended offset adjustment circuit is shown in
IN
3
Figure 3.
4
10µF ^0.1µF
-5V
500
8
7
6
5
GND
V_H
0.1µF
50Ω
10µF
+5V
OUT
V_L
GND
FIGURE 2. EVALUATION BOARD SCHEMATIC AND LAYOUT
510
2-
6
HFA1120
V_OUT
V_IN
3+
5
1
4 _10k
V-
FIGURE 3. OFFSET VOLTAGE ADJUSTMENT CIRCUIT
3-213
Spec Number 511105-883