Datasheet

MAX9934
High-Precision, Low-Voltage, Current-Sense Amplifier
with Current Output and Chip Select for Multiplexing
15
Maxim Integrated
Three components are to be selected to optimize the
current-sense system: R
SENSE
, R
OUT
, and the
MAX9934 gain option (G
M
= 25µA/mV or 5µA/mV).
Tables 1 and 2 are gain tables for unidirectional and
bidirectional operation, respectively. They offer a few
examples for both MAX9934 options having an output
range of 3.1V unidirectional and ±1.65V bidirectional.
Note that the output current of the MAX9934 adds to its
quiescent current. This can be calculated as follows:
I
OUT,MAX
= V
OUT,MAX
/R
OUT
When selecting R
SENSE
, consider the expected magni-
tude of I
LOAD
and the required V
SENSE
to manage
power dissipation in R
SENSE
:
R
SENSE
= V
SENSE,MAX
/I
LOAD,MAX
R
SENSE
is typically a low-value resistor specifically
designed for current-sense applications.
Finally, in selecting the appropriate MAX9934 gain option
(G
M
), consider both the required V
SENSE
and I
OUT
:
G
M
= I
OUT,MAX
/V
SENSE,MAX
Once all three component values have been selected in
the current-sense application, the system performance
is represented by:
V
SENSE
= R
SENSE
x I
LOAD
and
V
OUT
= V
SENSE
x G
M
x R
OUT
Accuracy
In a first-order analysis of accuracy there are two
MAX9934 specifications that contribute to output error,
input offset (V
OS
) and gain error (GE). The MAX9934 has
a maximum V
OS
of 10µV and a maximum GE of 0.25%.
Note that the tolerance and temperature coefficient of
the chosen resistors directly affect the precision of any
measurement system.
Efficiency and Power Dissipation
At high-current levels, the I
2
R losses in R
SENSE
can be
significant. Take this into consideration when choosing
the resistor value and its power dissipation (wattage)
rating. Also, the sense resistor’s value drifts if it is
allowed to self-heat excessively. The precision V
OS
of
the MAX9934 allows the use of a small sense resistor to
reduce power dissipation and eliminate hot spots.
Kelvin Contacts
Due to the high currents that flow through R
SENSE
, take
care to prevent trace resistance in the load current path
from causing errors in the sense voltage. Use a four ter-
minal current-sense resistor or Kelvin contacts (force
and sense) PCB layout techniques.
Interfacing the MAX9934 to SAR ADCs
Since the MAX9934 is essentially a high-output imped-
ance current-source, its output termination resistor,
R
OUT
, acts like a source impedance when driving an
ADC channel. Most successive approximation register
(SAR) architecture ADCs specify a maximum source
resistance to avoid compromising the accuracy of their
readings. Choose the output termination resistor R
OUT
such that it is less than that required by the ADC speci-
fication (10kΩ or less). If the R
OUT
is larger than the
source resistance specified, the ADC internal sampling
capacitor can momentarily load the amplifier output
and cause a drop in the voltage reading.
If R
OUT
is larger than the source resistance specified,
consider using a ceramic capacitor from ADC input to
GND. This input capacitor supplies momentary charge
to the internal ADC sampling capacitor, helping hold
V
OUT
constant to within ±1/2 LSB during the acquisition
period. Use of this capacitor reduces the noise in the
output signal to improve sensitivity of measurement.
PART
V
SENSE
(mV)
OUTPUT
CURRENT
(µA)
R
OUT
(kΩ)
GAIN
(V/V)
12.4 310 10 250
MAX9934T
24.8 620 5 125
62 310 10 50
MAX9934F
75 375 8 40
Table 1. Unidirectional Gain Table*
* All calculations were made with V
CC
= 3.3V and V
OUT(MAX)
=
V
CC
- V
OH
= 3.1V.
PART
V
SENSE
(mV)
OUTPUT
CURRENT
(µA)
R
OUT
(kΩ)
GAIN (V/V)
±5.8 ±145 10 250
±11.6 ±290 5 125
MAX9934T
±24 ±600 2.4 60
±29 ±145 10 50
±58 ±290 5 25
MAX9934F
±72 ±360 4 20
Table 2. Bidirectional Gain Table*
* All calculations were made with V
CC
= 3.3V, V
OUT(MAX)
= V
CC
-
V
OH
= 3.1V, V
OUT(MIN)
= V
OL
, and OUT connected to an exter-
nal reference voltage of V
REF
= 1.65V through R
OUT
.