ADA4661-2ARMZ-R7 datasheet(22/32 Pages) AD

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ADA4661-2

Data Sheet

Rev. 0 | Page 22 of 32

APPLICATIONS INFORMATION

Figure 70. Simplified Schematic

The ADA4661-2 is a low power, rail-to-rail input and output,

precision CMOS amplifier that operates over a wide supply

voltage range of 3 V to 18 V. This amplifier uses the Analog

Devices DigiTrim technique to achieve a higher degree of

precision than is available from other CMOS amplifiers. The

DigiTrim technique is a method of trimming the offset voltage

of an amplifier after assembly. The advantage of postpackage

trimming is that it corrects any offset voltages caused by

mechanical stresses of assembly. To achieve a rail-to-rail input

and output range with very low supply current, the ADA4661-2

uses unique input and output stages.

INPUT STAGE

Figure 70 shows the simplified schematic of the ADA4661-2.

The amplifier uses a three-stage architecture with a fully

differential input stage to achieve excellent dc performance

specifications.

The input stage comprises two differential transistor pairs—

a NMOS pair (M1, M2) and a PMOS pair (M3, M4)—and

folded-cascode transistors (M5 to M12). The input common-

mode voltage determines which differential pair is active. The

PMOS differential pair is active for most of the input common-

mode range. The NMOS pair is required for input voltages up

to and including the upper supply rail. This topology allows the

amplifier to maintain a wide dynamic input voltage range and

maximize signal swing to both supply rails.

The proprietary high voltage protection circuitry in the

ADA4661-2 minimizes the common-mode voltage changes

seen by the amplifier input stage for most of the input common-

mode range. This results in the amplifier having excellent

disturbance rejection when operating in this preferred

common-mode range. The performance benefits of operating

within this preferred range are shown in the PSRR vs. VCM (see

Figure 17), CMRR vs. VCM (see Figure 14), and VOS vs. VCM

graphs (see Figure 8, Figure 11, Figure 12, Figure 13, Figure 15,

and Figure 16). The CMRR performance benefits of the reduced

common-mode range are guaranteed at final test and shown in the

electrical characteristics (see Table 2 to Table 4).

For most of the input common-mode voltage range, the PMOS

differential pair is active. When the input common-mode

voltage is within a few volts of the power supplies, the input

transistors are exposed to these voltage changes. As the

common-mode voltage approaches the positive power supply,

the active differential pair changes from the PMOS pair to the

NMOS pair. Differential pairs commonly exhibit different offset

voltages. The handoff of control from one differential pair to the

other creates a step like characteristic that is visible in the VOS vs.

VCM graphs (see Figure 8, Figure 11, Figure 12, Figure 13, Figure 15,

and Figure 16). This characteristic is inherent in all rail-to-rail

input amplifiers that use the dual differential pair topology.

Additional steps in the VOS vs. VCM graphs are visible as the

common-mode voltage approaches the negative power supply.

These changes are a result of the load transistors (M5, M6)

running out of headroom. As the load transistors are forced into

the triode region of operation, the mismatch of their drain

impedance becomes a significant portion of the amplifier offset.

This effect can also be seen in the VOS vs. VCM graphs (see Figure 8,

Figure 11, Figure 12, Figure 13, Figure 15, and Figure 16).

Current Source I2 drives the PMOS transistor pair. As the input

common-mode voltage approaches the upper power supply,

this current is reduced to zero. At the same time, a replica

current source, I1, is increased from zero to enable the NMOS

transistor pair.

The ADA4661-2 achieves its high performance specifications by

using low voltage MOS devices for its differential inputs. These

low voltage MOS devices offer excellent noise and bandwidth

per unit of current. The input stage is isolated from the high

V+

V–

+IN x

OUT x

M11

M12

M10

HIGH VOLTAGE PROTECTION

M15

M21

M22

M16

M13

M14

M19

M20

M17

M18

–IN x

HIGH VOLTAGE PROTECTION

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