AD5522JSVUZ datasheet(32/65 Pages) AD

32 / 65 page

Data Sheet

AD5522

Rev. E | Page 31 of 64

When the AD5522 is placed in high-Z mode, the clamp circuit

is always configured to monitor the measure current signal

(irrespective of which high-Z mode is selected, high-Z V or

high-Z I). At this time, the clamp circuit is also comparing to

the voltage clamp levels. Because the device is in high-Z mode,

the measure current signal is at zero, but zero for the measure

current is always the VMID voltage set by the offset DAC. For

default offset DAC conditions, this causes no concern. For other

settings of the offset DAC, the zero point follows the VMID,

and as the clamp circuit is comparing the voltage clamp levels

to the measure current signal, there may be instances where

the voltage clamp levels cause the alarm to flag during high-Z

mode. To avoid this, the clamps can be disabled when going

into high-Z mode.

CURRENT RANGE SELECTION

Integrated thin film resistors minimize the need for external

components and allow easy selection of any of these current

ranges: ±5 µA (200 kΩ), ±20 μA (50 kΩ), ±200 μA (5 kΩ), and

±2 mA (500 Ω). One current range up to ±80 mA can be

accommodated per channel by connecting an external sense

resistor. For current ranges in excess of ±80 mA, it is necessary

that an external amplifier be used.

For the suggested current ranges, the maximum voltage drop

across the sense resistors is ±1 V. However, to allow for error

correction, there is some overrange available in the current

ranges (±12.5% or ±0.125 V across RSENSE). The full-scale

voltage range that can be loaded to the FIN DAC is ±11.5 V;

the forced current can be calculated as follows:

FI = 4.5 × VREF × ((DAC_CODE − 32,768)/216)/(RSENSE ×

MI_Amplifier_Gain)

where:

FI is the forced current.

RSENSE is the selected sense resistor.

MI_Amplifier_Gain is the gain of the measure current

instrumentation amplifier. This gain can be set to 5 or 10

via the serial interface.

In the ±200 μA range with the 5 kΩ sense resistor and an ISENSE

gain of 10, the maximum current range possible is ±225 μA.

Similarly, for the other current ranges, there is an overrange of

12.5% to allow for error correction.

Also, the forced current range is the quoted full-scale range only

with an applied reference of 5 V or 2.5 V (with ISENSE amplifier

gain = 5). The ISENSE amplifier is biased by the VMID DAC

voltage in such a way as to center the measure current output

irrespective of the voltage span used.

When using the EXTFOHx outputs for current ranges up to

±80 mA, there is no switch in series with the EXTFOHx line,

ensuring minimum capacitance present at the output of the

force amplifier. This feature is important when using a pin

electronics driver to provide high current ranges.

HIGH CURRENT RANGES

With the use of an external high current amplifier, one high

current range in excess of ±80 mA is possible. The high current

amplifier buffers the force output and provides the drive for the

required current.

To eliminate any timing concerns when switching between the

internal ranges and the external high current range, there is a

mode where the internal ±80 mA stage can be enabled at all

times. See Table 26 for more information.

DUTGND

MEASVHx

DUT

RSENSE

EXTMEASIHx

EXTMEASILx

FOHx

EXTFOHx

HIGH

CURRENT

AMPLIFIER

CFFx

FIN

INTERNAL RANGE SELECT

(±5µA, ±20µA, ±200µA, ±2mA)

DAC

–

×5 or ×10

×1

AGND

MEASOUTx

×1 OR

×0.2

Ω

2kΩ

VMID TO

CENTER

I RANGE

Ω

Figure 47. Addition of High Current Amplifier for Wider Current Range (>±80 mA)

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