ADA4530-1ARZ-R7 datasheet(28/52 Pages) AD

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Data Sheet

ADA4530-1

THEORY OF OPERATION

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Rev. C | 28 of 52

The ADA4530-1 is an operational amplifier designed to interface

with the extremely high impedance sensors used in electrometer

applications.

A metal-oxide semiconductor field effect transistor (MOSFET) in-

put stage eliminates the gate leakage currents associated with

legacy junction gate field effect transistor (JFET) electrometers.

The ADA4530-1 achieves extremely low input bias currents while

simultaneously providing robust protection against ESD damage.

A unique ESD diode structure provides protection while also allow-

ing the diodes to be guarded to minimize leakage currents to

the input pins. The ADA4530-1 integrates the precision buffer that

guards internal ESD diode leakage paths. The output of this guard

buffer also connects to external pins, allowing the user to guard

external components against leakage currents.

The input bias current is determined by the accuracy of the guard

voltage applied across the ESD diodes. The offset voltages of the

amplifier and guard buffer set the accuracy of the guard voltage

and, therefore, the input bias current.

The ADA4530-1 uses Analog Devices, Inc., DigiTrim® technology to

achieve superior performance.

DigiTrim trims the offset voltage of the amplifier and guard buffer

to reject changes in the common-mode voltage, power supply volt-

age, and temperature. This technique significantly improves VOS,

CMRR, PSRR, and offset voltage temperature coefficient (TCVOS)

specifications.

Figure 99 shows the simplified schematic of the ADA4530-1. The

amplifier uses a three-stage architecture with a fully differential

input stage to achieve excellent dc performance specifications.

ESD STRUCTURE

The input ESD structure consists of Diode D1 to Diode D6.

The noninverting input is coupled to the guard pins (GRD) by the

D1 and D2 antiparallel diodes. The inverting input is coupled to the

guard pins by the D3 and D4 antiparallel diodes.

The guard pins are connected to the power supplies through Di-

ode D5 and Diode D6. During ESD events, the transient current

flows from the input pins through one of the antiparallel diodes

and harmlessly into the supplies through one of the power supply

diodes. During normal operation, the guard buffer (BUF1) forces the

voltage across the antiparallel diodes to 0 V. Resistor R1 shields

the guard buffer from potentially large capacitances connected to

the guard pins. Its value is nominally 1 kΩ.

INPUT STAGE

The input stage comprises a P-channel metal-oxide semiconductor

(PMOS) differential pair (M1, M2), folded cascode transistors (M5 to

M12), and current source (I1).

The ADA4530-1 achieves its high performance specifications by

using low voltage MOS devices for its differential inputs. These

low voltage MOS devices offer better 1/f noise and bandwidth per

unit current compared to high voltage devices. The input stage is

isolated from the high system voltages with proprietary protection

circuitry. This regulation circuitry protects the input devices from the

high supply voltages in which the amplifier can operate.

The proprietary high voltage protection circuitry in the ADA4530-1

operates to minimize the common-mode voltage changes seen by

the amplifier input stage for most of the input common-mode range.

This circuitry results in excellent disturbance rejection when operat-

ing in this preferred input common-mode range. The performance

benefits of operating within this preferred range are shown in the

VOS vs. VCM graphs (see Figure 16 to Figure 18), the small signal

CMRR vs. VCM graph (see Figure 21), and the small signal PSRR

vs. VCM graph (see Figure 54).

The input devices are protected from large differential input voltag-

es by the antiparallel ESD diodes (D1 to D4). The diodes can

conduct significant current when the differential voltage exceeds

700 mV. The user must ensure that the current flowing into the input

pins is limited to the absolute maximum of 10 mA.

Figure 99. Simplified Schematic

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