AD627

–13–

REV. A

had a mismatch of almost exactly 0.1% (R1 = 9999.5

Ω, R2 =

999.76

Ω, R3 = 1000.2 Ω, R4 = 9997.7 Ω). As expected the

CMRR at dc was measured at about 84 dB (calculated value

is 85 dB). However, as the frequency increases, the CMRR

quickly degrades. For example, a 200 mV peak-peak harmonic

of the mains frequency at 180 Hz would result in an output

voltage of about 800

µV. To put this in context, a 12-bit data

acquisition system with an input range of 0 V to 2.5 V, has an

LSB weighting of 610

µV.

By contrast, the AD627 uses precision laser trimming of internal

resistors along with patented CMR trimming to yield a higher

dc CMRR and a wider bandwidth over which the CMRR is flat

(see Figure 20).

AD296

1/2

R2

999.76

R1

9999.5

R3

1000.2

R4

9997.7

+5V

VIN+

VIN–

–5V

A1

A2

AD296

1/2

Figure 39. 0.1% Resistor Mismatch Example

+5V

40.2k

1%

+10ppm/ C

AD627A

2.5V

350

350

350

350

100mV

+5V

AD627A GAIN = 9.98

1/2

350 *

3.15k *

350 *

2.5V

3.15k *

+5V

1/2

"HOMEBREW" IN AMP, G = 10

*1% REGISTER MATCH, 50ppm/ C TRACKING

LT1078IS8

LT1078IS8

Figure 38. Make vs. Buy

FREQUENCY – Hz

120

1

110

100

90

80

70

60

50

40

30

20

10

100

1k

10k

100k

Figure 40. CMRR Over Frequency of Discrete In Amp in

Figure 39

Ground Returns for Input Bias Currents

Input bias currents are those dc currents that must flow in

order to bias the input transistors of an amplifier. These are

usually transistor base currents. When amplifying “floating”

input sources such as transformers, or ac-coupled sources,

there must be a direct dc path into each input in order that the

bias current can flow. Figure 41 shows how a bias current

path can be provided for the case of transformer coupling,

capacitive ac-coupling and for a thermocouple application.

Table IV. Make vs. Buy Error Budget

“Homebrew”

Total Error

Total Error

Error Source

AD627 Circuit Calculation

Circuit Calculation

AD627-ppm

Homebrew–ppm

Total RTI Offset Voltage, mV

(250

µV + (1000 µV/10))/100 mV

(180

µV × 2)/100 mV

3500

3600

Input Offset Current, nA

1 nA

× 350 Ω/100 mV

20 nA

× 350 Ω/100 mV

3.5

70

Internal Offset Current (Homebrew Only)

Not Applicable

0.7 nA

× 350 Ω/100 mV

2.45

CMRR, dB

77 dB¡141 ppm

× 2.5 V/100 mV

(1% Match

× 2.5 V)/10/100 mV

3531

25000

Gain

0.35% + 0.1%

1% Match

13500

10000

Total Absolute Error

20535

38672

DRIFT TO +85

°C

Gain Drift, ppm/

°C

(–75 + 10) ppm/

°C × 60°C

50 ppm/

°C × 60°C

3900

3000

Total RTI Offset Voltage, mV/

°C

(3.0

µV/°C + (10 µV/°C/10))

(2

× 3.5 µV/°C × 60°C)/100 mV

× 60°C/100 mV

2600

4200

Input Offset Current, pA/

°C

(16 pA/

°C × 350 Ω × 60°C)/100 mV (33 pA/°C × 350 Ω × 60°C)/100 mV

3.5

7

Total Drift Error

6504

7207

Grand Total Error

27039

45879