AD8620BRZ(RevE) データシートの表示（PDF） - Analog Devices

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AD8620BRZ
(Rev.:RevE)

Precision, Very Low Noise, Low Input Bias Current, Wide Bandwidth JFET Operational Amplifier

Analog Devices 

Input Offset Voltage Adjustment

Offset of AD8610 is very small and normally does not require

additional offset adjustment. However, the offset adjust pins can

be used as shown in Figure 66 to further reduce the dc offset. By

using resistors in the range of 50 kΩ, offset trim range is ±3.3 mV.

+VS

7

2

AD8610 6

1

3

5

4

R1

VOUT

–VS

Figure 66. Offset Voltage Nulling Circuit

Programmable Gain Amplifier (PGA)

The combination of low noise, low input bias current, low input

offset voltage, and low temperature drift make the AD8610/

AD8620 a perfect solution for programmable gain amplifiers.

PGAs are often used immediately after sensors to increase the

dynamic range of the measurement circuit. Historically, the large

on resistance of switches (combined with the large IB currents

of amplifiers) created a large dc offset in PGAs. Recent and

improved monolithic switches and amplifiers completely remove

these problems. A PGA discrete circuit is shown in Figure 67.

In Figure 67, when the 10 pA bias current of the AD8610 is

dropped across the (<5 Ω) RON of the switch, it results in a

negligible offset error.

When high precision resistors are used, as in the circuit of

Figure 67, the error introduced by the PGA is within the

½ LSB requirement for a 16-bit system.

AD8610/AD8620

5V

7

100Ω

VIN

3

1

AD8610 6

2

5

VOUT

4

10kΩ

5pF

+5V

+5V +5V

12

13

VL VDD

S1 3

1

Y0

G

IN1

D1 2

ADG452 S2 14

16

Y1

IN2

A0 A

A1 B

9

Y2

IN3

D2 15

S3 11

D3 10

S4 6

8

Y3

74HC139

IN4

D4 7

VSS GND

4

5

1kΩ

G = +1

10kΩ

G = +10

1kΩ

G = +100

100Ω

G = +1000

11Ω

–5V

Figure 67. High Precision PGA

1. Room temperature error calculation due to RON and IB:

ΔVOS = IB × RON = 2 pA × 5 Ω = 10 pV

Total Offset = AD8610 (Offset) + ΔVOS

Total Offset = AD8610 (Offset_Trimmed) + ΔVOS

Total Offset = 5 μV + 10 pV ≅ 5 μV

2. Full temperature error calculation due to RON and IB:

ΔVOS (@ 85°C) = IB (@ 85°C) × RON (@ 85°C) =

250 pA × 15 Ω = 3.75 nV

3. Temperature coefficient of switch and AD8610/AD8620

combined is essentially the same as the TCVOS of the

AD8610/AD8620:

ΔVOS/ΔT(total) = ΔVOS/ΔT(AD8610/AD8620) +

ΔVOS/ΔT(IB × RON)

ΔVOS / ΔT(total) = 0.5 μV /°C + 0.06 nV/°C ≅ 0.5 μV /°C

Rev. E | Page 19 of 24

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