AD8620 datasheet(13/24 Pages) AD

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AD8610/AD8620

Rev. E | Page 13 of 24

THEORY OF OPERATION

–

VIN

20V p-p

+13V

–13V

2kΩ

20kΩ

2kΩ

V+

V–

V+

CS (dB) = 20 log (VOUT / 10 × VIN)

Figure 41. Channel Separation Test Circuit

FUNCTIONAL DESCRIPTION

The AD8610/AD8620 are manufactured on Analog Devices,

Inc.'s XFCB (eXtra fast complementary bipolar) process. XFCB

is fully dielectrically isolated (DI) and used in conjunction with

N-channel JFET technology and thin film resistors (that can be

trimmed) to create the JFET input amplifier. Dielectrically iso-

lated NPN and PNP transistors fabricated on XFCB have an FT

greater than 3 GHz. Low TC thin film resistors enable very accurate

offset voltage and offset voltage tempco trimming. These process

breakthroughs allow Analog Devices’ IC designers to create an

amplifier with faster slew rate and more than 50% higher band-

width at half of the current consumed by its closest competition.

The AD8610/AD8620 are unconditionally stable in all gains,

even with capacitive loads well in excess of 1 nF. The AD8610/

AD8620B grade achieves less than 100 μV of offset and 1 μV/°C

of offset drift, numbers usually associated with very high precision

bipolar input amplifiers. The AD8610 is offered in the tiny 8-lead

MSOP as well as narrow 8-lead SOIC surface-mount packages

and is fully specified with supply voltages from ±5 V to ±13 V.

The very wide specified temperature range, up to 125°C, guarantees

superior operation in systems with little or no active cooling.

The unique input architecture of the AD8610/AD8620 features

extremely low input bias currents and very low input offset volt-

age. Low power consumption minimizes the die temperature and

maintains the very low input bias current. Unlike many com-

petitive JFET amplifiers, the AD8610/AD8620 input bias currents

are low even at elevated temperatures. Typical bias currents are

less than 200 pA at 85°C. The gate current of a JFET doubles

every 10°C resulting in a similar increase in input bias current

over temperature. Give special care to the PC board layout to

minimize leakage currents between PCB traces. Improper lay-

out and board handling generates a leakage current that exceeds

the bias current of the AD8610/AD8620.

138

136

120

128

126

124

122

132

130

134

FREQUENCY (kHz)

100

150

200

250

300

350

Figure 42. AD8620 Channel Separation Graph

TEMPERATURE (°C)

–75

–25

–50

100

125

OPA627

AD8610

Figure 43. Supply Current vs. Temperature

Power Consumption

A major advantage of the AD8610/AD8620 in new designs is

the power saving capability. Lower power consumption of the

AD8610/AD8620 makes them much more attractive for portable

instrumentation and for high density systems, simplifying ther-

mal management, and reducing power-supply performance

requirements. Compare the power consumption of the AD8610

vs. the OPA627 in Figure 43.

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