AD8620BR datasheet(19/24 Pages) AD

Il numero della parte	AD8620BR

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

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Produttore elettronici	AD [Analog Devices]

Homepage	http://www.analog.com

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19 / 24 page

AD8610/AD8620

Rev. E | Page 19 of 24

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.

R1

+VS

VOUT

–VS

AD8610

7

6

1

5

4

3

2

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.

VIN

VOUT

AD8610

7

4

6

5

1

2

3

IN1

S1

D1

10kΩ

1kΩ

5V

+5V

IN2

S2

D2

IN3

S3

D3

IN4

S4

D4

ADG452

3

2

14

15

11

10

6

7

VL

VDD

13

12

1

16

9

8

74HC139

VSS

4

GND

5

1kΩ

100Ω

11Ω

5pF

G = +1

G = +10

G = +100

G = +1000

+5V

–5V

Y0

Y1

Y2

Y3

G

A

B

A0

A1

100Ω

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

μV

5

pV

10

μV

5

≅

+

=

Offset

Total

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)

C

V

C

total

T

V

OS

°

μ

≅

°

+

°

=

Δ

/

5

.

0

nV/

06

.

0

/

μV

5

.

0

)

(

/

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