AD7811/AD7812
An example of the pseudo differential scheme using the AD7811
is shown in Figure 6. The relevant bits in the AD7811 Control
Register are set as follows DIF/SGL = 1, CH1 = CH2 = 0, i.e.,
VIN1 pseudo differential with respect to VIN2. The signal is
applied to VIN1 but in the pseudo differential scheme the sam-
pling capacitor is connected to VIN2 during conversion and not
AGND as described in the Converter Operation section. This
input scheme can be used to remove offsets that exist in a sys-
tem. For example, if a system had an offset of 0.5 V the offset
could be applied to VIN2 and the signal applied to VIN1. This has
the effect of offsetting the input span by 0.5 V. It is only pos-
sible to offset the input span when the reference voltage is less
than VDD–OFFSET.
Figure 8 shows the equivalent charging circuit for the sampling
capacitor when the ADC is in its acquisition phase. R2 repre-
sents the source impedance of a buffer amplifier or resistive
network; R1 is an internal multiplexer resistance, and C1 is the
sampling capacitor. During the acquisition phase the sampling
capacitor must be charged to within a 1/2 LSB of its final value.
The time it takes to charge the sampling capacitor (TCHARGE) is
given by the following formula:
T
CHARGE = 7.6 × (R2 + 125 Ω) × 3.5 pF
R1
125⍀
V
IN+
R2
SAMPLING
CAPACITOR
C1
3.5pF
CHARGE
REDISTRIBUTION
DAC
Figure 8. Equivalent Sampling Circuit
SAMPLING
For small values of source impedance, the settling time associ-
ated with the sampling circuit (100 ns) is, in effect, the acquisi-
tion time of the ADC. For example, with a source impedance
(R2) of 10 Ω the charge time for the sampling capacitor is
approximately 4 ns. The charge time becomes significant for
source impedances of 2 kΩ and greater.
V
CAPACITOR
V
V
IN1
IN2
IN+
V
IN1
CONTROL
LOGIC
V
OFFSET
CONVERSION
PHASE
COMPARATOR
IN–
CLOCK
OSC
V
V
OFFSET
V
/3
DD
AC Acquisition Time
In ac applications it is recommended to always buffer analog
input signals. The source impedance of the drive circuitry must
be kept as low as possible to minimize the acquisition time of
the ADC. Large values of source impedance will cause the THD
to degrade at high throughput rates. In addition, better perfor-
mance can generally be achieved by using an External 1 nF
capacitor on VIN.
Figure 6. Pseudo Differential Input Scheme
When using the pseudo differential input scheme the signal on
IN2 must not vary by more than a 1/2 LSB during the conver-
sion process. If the signal on VIN2 varies during conversion, the
conversion result will be incorrect. In single-ended mode the
sampling capacitor is always connected to AGND during con-
version. Figure 7 shows the AD7811/AD7812 pseudo differen-
tial input being used to make a unipolar dc current measurement.
A sense resistor is used to convert the current to a voltage and
the voltage is applied to the differential input as shown.
V
ON-CHIP REFERENCE
The AD7811 and AD7812 have an on-chip 2.5 V reference
circuit. The schematic in Figure 9 shows how the reference
circuit is implemented. A 1.23 V bandgap reference is gained up
to provide a 2.5 V 2% reference voltage. The on-chip refer-
ence is not available externally (SW2 is open). An external refer-
ence (1.2 V to VDD) can be applied at the VREF pin. However in
order to use an external reference the EXTREF bit in the con-
trol register (Bit 0) must first be set to a Logic 1. When EXTREF
is set to a Logic 1 SW2 will close, SW3 will open and the ampli-
fier will power down. This will reduce the current consumption
of the part by about 1 mA. It is possible to use two different
reference voltages by selecting the on-chip reference or external
reference.
V
DD
V
IN+
AD7811/
AD7812
R
SENSE
V
IN–
R
L
Figure 7. DC Current Measurement Scheme
DC Acquisition Time
C
REF
The ADC starts a new acquisition phase at the end of a conver-
sion and ends on the falling edge of the CONVST signal. At the
end of a conversion a settling time is associated with the sam-
pling circuit. This settling time lasts approximately 100 ns. The
analog signal on VIN+ is also being acquired during this settling
time. Therefore, the minimum acquisition time needed is
approximately 100 ns.
EXTERNAL
CAPACITOR
V
REF
SW1
SW2
1.23V
2.5V
7pF
SW3
AGND
Figure 9. On-Chip Reference Circuitry
REV. B
–11–
ADI [ ADI ]
