ADA4805-1/ADA4805-2

Data Sheet

Rev. B | Page 20 of 25

APPLICATIONS INFORMATION

SLEW ENHANCEMENT

The ADA4805-1/ADA4805-2 have an internal slew enhancement

circuit that increases the slew rate as the feedback error voltage

increases. This circuit allows the amplifier to settle a large step

response faster, as shown in Figure 56. This is useful in ADC

applications where multiple input signals are multiplexed. The

impact of the slew enhancement can also be seen in the large

signal frequency response, where larger input signals cause a

slight increase in peaking, as shown in Figure 57.

TIME (ns)

50

40

30

20

10

0100

90

80

70

60

–0.5

–1.0

–1.5

0.5

0

1.0

1.5

G = +1

Figure 56. Step Response with Selected Output Steps

–6

–5

–4

–3

–2

–1

0

1

2

FREQUENCY (Hz)

G = +1

100k

1M

10M

100M

Figure 57. Peaking in Frequency Responses as Signal Level Changes, G = +1

EFFECT OF FEEDBACK RESISTOR ON FREQUENCY

RESPONSE

The amplifiers input capacitance and feedback resistor form a

pole that, for larger value feedback resistors, can reduce phase

margin and contribute to peaking in the frequency response.

when the amplifier is configured in a gain of +2. Figure 58 also

shows how peaking can be mitigated with the addition of a

small value capacitor placed across the feedback resistor of the

amplifier.

–6

–5

–4

–3

–2

–1

0

1

2

3

4

5

FREQUENCY (Hz)

100k

1M

10M

100M

G = +2

COMPENSATING PEAKING IN LARGE SIGNAL

FREQUENCY RESPONSE

At high frequency, the slew enhancement circuit can contribute to

peaking in the large signal frequency response. Figure 58 shows the

effect of a feedback capacitor on the small signal response, whereas

Figure 59 shows that the same technique is effective for reducing

peaking in the large signal response.

–15

–12

–9

–6

–3

0

3

6

FREQUENCY (Hz)

G = +2

100k

1M

10M

100M

Figure 59. Peaking Mitigation in Large Signal Frequency Response

DRIVING LOW POWER, HIGH RESOLUTION

SUCCESSIVE APPROXIMATION REGISTER (SAR)

ADCs

The ADA4805-1/ADA4805-2 are ideal for driving low power,

high resolution SAR ADCs. The 5.9 nV/√Hz input voltage noise

and rail-to-rail output stage of the ADA4805-1/ADA4805-2

help to minimize distortion at large output levels. With its low

power of 500 μA, the amplifier consumes power that is

compatible with low power SAR ADCs, which are usually in the

microwatt (μW) to low milliwatt (mW) range. Furthermore, the

ADA4805-1/ADA4805-2 support a single-supply configuration;

their input common-mode range extends to 0.1 V below the

negative supply, and 1 V below the positive supply.