AD603ACHIPS datasheet(13/20 Pages) AD

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AD603

Rev. G | Page 13 of 20

2.15k

Ω

5.6pF

GPOS

GNEG

VINP

COMM

VPOS

VOUT

VNEG

FDBK

AD603

VC1

VC2

VIN

VPOS

VOUT

VNEG

Figure 31. 0 dB to 40 dB; 30 MHz Bandwidth

18pF

GPOS

GNEG

VINP

COMM

VPOS

VOUT

VNEG

FDBK

AD603

VC1

VC2

VIN

VPOS

VOUT

VNEG

Figure 32. 10 dB to 50 db; 9 MHz to Set Gain

REXT (Ω)

100

10k

100k

–1:VdB (OUT)

–2:VdB (OUT)

VdB (OUT)

Figure 33. Gain vs. REXT, Showing Worst-Case Limits Assuming Internal

Resistors have a Maximum Tolerance of 20%

Optionally, when a resistor is placed from FDBK to COMM,

higher gains can be achieved. This fourth mode is of limited

value because of the low bandwidth and the elevated output

offsets; it is thus not included in Figure 30, Figure 31, or

Figure 32.

The gain of this amplifier in the first two modes is set by the

ratio of on-chip laser-trimmed resistors. While the ratio of these

resistors is very accurate, the absolute value of these resistors

can vary by as much as ±20%. Thus, when an external resistor is

connected in parallel with the nominal 6.44 kΩ ±20% internal

resistor, the overall gain accuracy is somewhat poorer. The

worst-case error occurs at about 2 kΩ (see Figure 34).

REXT (Ω)

100

10k

100k

1.2

1.0

0.8

0.6

0.4

0.2

–0.2

–0.4

–0.6

–0.8

–1.0

–1:VdB (OUT) – (–1):VdB (OREF)

VdB (OUT) – VdB (OREF)

Figure 34. Worst-Case Gain Error, Assuming Internal Resistors have a

Maximum Tolerance of −20% (Top Curve) or =20% (Bottom Curve)

While the gain bandwidth product of the fixed-gain amplifier is

about 4 GHz, the actual bandwidth is not exactly related to the

maximum gain. This is because there is a slight enhancing of

the ac response magnitude on the maximum bandwidth range,

due to higher order poles in the open-loop gain function; this

mild peaking is not present on the higher gain ranges. Figure 30,

Figure 31, and Figure 32 show how an optional capacitor may

be added to extend the frequency response in high gain modes.

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