ADA4807-2ACPZ-R2 datasheet(24/33 Pages) AD

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ADA4807-1/ADA4807-2/ADA4807-4

Data Sheet

Rev. B | Page 24 of 33

THEORY OF OPERATION

The ADA4807-1/ADA4807-2/ADA4807-4 have a rail-to-rail

input stage with an input range that goes 200 mV beyond either

rail. A PNP transistor input pair is active for a majority of the

input range, while an NPN transistor input pair is active for the

common-mode voltages within 1.3 V of the positive rail. The

ADA4807-1/ADA4807-2/ADA4807-4 are fabricated using the

Analog Devices, Inc., third generation, extra fast complementary

bipolar (XFCB) process resulting in exceptionally good distortion,

noise, slew rate, and settling characteristics for 1 mA devices.

Given traditional rail-to-rail input architecture performance, the

input 1/f noise is surprisingly low, and the current noise is only

0.7 pA/√Hz for a 3 nV/√Hz voltage noise. Typical high slew rate

devices suffer from increased current noise because of input pair

degeneration and higher input stage current. The ADA4807-1/

ADA4807-2/ADA4807-4 exceed current benchmark parameters

given the performance of the XFCB process.

The multistage design of the ADA4807-1/ADA4807-2/

ADA4807-4 has excellent precision specifications, such as input

drift, offset, open-loop gain, CMRR, and PSRR. Typical harmonic

distortion numbers fall in the range of −130 dBc for a 10 kHz

fundamental (see the Distortion and Noise section). This level of

performance makes the ADA4807-1/ADA4807-2/ADA4807-4 the

best choices when driving 18-bit precision converters.

The ADA4807-1/ADA4807-2 are optimized for a low shutdown

current (4 μA maximum), in the order of a few microamperes. In

power sensitive applications, this can eliminate the use of a power

FET and enable time interleaved power saving operation schemes.

The rail-to-rail input stage is useful in many different applications.

Although the precision is reduced from input to input, many

applications can tolerate this loss when the alternative is no

functionality at all. The positive rail input range is indispensable

for servo loops with a high-side input range

The ADA4807-1/ADA4807-2/ADA4807-4 input operates 200 mV

beyond either rail. Internal protection circuitry prevents the output

from phase inverting when the input range is exceeded. When

the input exceeds a diode beyond either rail, internal electrostatic

discharge (ESD) protection diodes source or sink current through

the input.

DIFFERENTIAL

DRIVE

FROM

INPUT STAGE

Q37

Q47

Q21

Q20

Q51

Q27

Q68

Q44

Q42

Q48

Q49

Q50

Q43

VOUT

Q38

R29

Figure 62. Differential Drive from Input Stage

Q13

Q17

Q14

Q11

Q18

+VS

VIN

VIP

–VS

OUTPUT STAGE,

COMMON-MODE

FEEDBACK

VBIAS1

VBIAS2

5µA

1.3V

Figure 63. Simplified Schematic

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