MAX5420/MAX5421
Detailed Description
The MAX5420/MAX5421 are digitally programmable pre-
cision voltage dividers for programmable-gain amplifiers.
Operationally, these devices consist of digitally selectable
precision resistor-dividers providing noninverting gains of
1, 2, 4, and 8 for PGA applications (see Functional
Diagram and Figure 1).
The MAX5420/MAX5421 achieve divider ratio accuracy of
0.025% (MAX5420A/MAX5421A), 0.09% (MAX5420B/
MAX5421B) or 0.5% (MAX5420C/MAX5421C).
The end-to-end resistance from H to L is 15kΩ. The
impedance seen at W is designed to be the same 8kΩ
for divider ratios 2, 4, and 8, ensuring excellent imped-
ance matching and constant switching times. In gain of
+1 configuration, H is internally connected to W with a
typical resistance of 500Ω.
Matching Resistor (MAX5421)
The MAX5421 includes a matching resistor to compen-
sate for offset voltage due to input bias current of the
op amp. The resistance from MATCH_H to MATCH_L,
equals the resistance seen at W for gains of 2, 4, and 8.
In the gain of +1, an internal switch connects
MATCH_H and MATCH_L. This internal switch matches
the impedance of the switch between H and W.
Digital Interface Operation
The MAX5420/MAX5421 feature a simple two-bit paral-
lel programming interface. D1 and D0 program the
divider ratio setting according to the Logic Control
Truth Table (Table 1). The digital interface is CMOS/TTL
logic compatible.
Applications Information
Programmable-Gain Amplifier
The MAX5420/MAX5421 are ideally suited for high-pre-
cision PGA applications. The typical application circuit
of Figure 3 uses the MAX5421 with matching resistor to
compensate for voltage offset due to op amp input bias
currents. Use the MAX5420 with an ultra-low input bias
current op amp such as the MAX4237 (Figure 4).
Power Supplies and Bypassing
The MAX5420/MAX5421 operate from dual ±5V sup-
plies, or a single +5V supply. For dual supplies, bypass
V
DD
and V
SS
with 0.1µF ceramic capacitors to GND.
For single supply, connect V
SS
to GND and bypass
V
DD
with a 0.1µF ceramic capacitor to GND.
Digital Control Inputs. Drive D0 and D1 logic high or logic low to set the divider ratio.
