Datasheet AD828 (Analog Devices) - 10

AD828 THEORY OF OPERATION Circuit Board Layout
The AD828 is a low cost, dual video operational amplifier Input and output runs should be laid out so as to physically designed to excel in high performance, high output current isolate them from remaining runs. In addition, the feedback video applications. resistor of each amplifier should be placed away from the feed- The AD828 consists of a degenerated NPN differential pair back resistor of the other amplifier, since this greatly reduces driving matched PNPs in a folded-cascade gain stage (Figure 4). interamp coupling. The output buffer stage employs emitter followers in a class AB
Choosing Feedback and Gain Resistors
amplifier that delivers the necessary current to the load while To prevent the stray capacitance present at each amplifier’s maintaining low levels of distortion. summing junction from limiting its performance, the feedback The AD828 will drive terminated cables and capacitive loads of resistors should be ≤ 1 kΩ. Since the summing junction capaci- 10 pF or less. As the closed-loop gain is increased, the AD828 tance may cause peaking, a small capacitor (1 pF to 5 pF) may will drive heavier cap loads without oscillating. be paralleled with RF to neutralize this effect. Finally, sockets should be avoided, because of their tendency to increase interlead
+VS
capacitance.
Power Supply Bypassing
Proper power supply decoupling is critical to preserve the integrity of high frequency signals. In carefully laid out designs, decoupling capacitors should be placed in close proximity to the supply pins, while their lead lengths should be kept to a
OUTPUT
minimum. These measures greatly reduce undesired inductive
–IN
effects on the amplifier’s response. Though two 0.1 µF capacitors will typically be effective in decoupling the supplies, several capacitors of different values
+IN
can be paralleled to cover a wider frequency range.
PARALLEL AMPS PROVIDE 100 mA TO LOAD –VS
By taking advantage of the superior matching characteristics of the Figure 4. Simplified Schematic AD828, enhanced performance can easily be achieved by employ- ing the circuit in Figure 5. Here, two identical cells are paralleled
INPUT CONSIDERATIONS
to obtain even higher load driving capability than that of a single An input protection resistor (R amplifier (100 mA min guaranteed). R1 and R2 are included to IN in TPC 31) is required in circuits where the input to the AD828 will be subjected to transient or limit current flow between amplifier outputs that would arise in continuous overload voltages exceeding the ± 6 V maximum dif- the presence of any residual mismatch. ferential limit. This resistor provides protection for the input transistors by limiting their maximum base current.
1k +VS 1 F
For high performance circuits, the “balancing” resistor should be
0.1 F
used to reduce the offset errors caused by bias current flowing
1k
through the input and feedback resistors. The balancing resistor
2 8 R1 5
equals the parallel combination of R
1/2
IN and RF and thus provides
1 AD828
a matched impedance at each input terminal. The offset voltage
3
error will then be reduced by more than an order of magnitude.
VIN VOUT APPLYING THE AD828 5 R2 R
The AD828 is a breakthrough dual amp that delivers precision and
5 1/2 L 7
speed at low cost with low power consumption. The AD828 offers
AD828 1k
excellent static and dynamic matching characteristics, combined
6 4 0.1 F
with the ability to drive heavy resistive loads.
1 F
As with all high frequency circuits, care should be taken to main- tain overall device performance as well as their matching. The
1k –VS
following items are presented as general design considerations. Figure 5. Parallel Amp Configuration –10– REV. C